In your climate you would either require a drainback system or ample antifreeze in the system to prevent freezing. A 50/50 mix of anti-freeze will be expensive but then again so will a lot of the other components.

 You could design solar collectors to heat the water on nice, sunny days. Using a radiator for heat to air transfer may require a larger radiator than what you have pictured. (or more of them). A hot water system is a lower grade heat than a fire. so the output of air temp is lower. This is why most hot water systems use radiant floors.

 Of course any piping running to the house needs to be VERY well insulated to keep losses down. Do remember that water expands with heat, so a low pressure or open system would be the best in this application. The radiator you show won't take any high amounts of pressure.

 It may be better to use a system with a large inground storage tank which stores heated water from your wood burner and any solar that you wish to add then install a heat exchanger in the tank to transfer the heat to your house and shed.

 that way you could easily automate a lot of the functions you'd have pump loops form your heat source to storage tanks, then a pump loop to the house which could be controlled directly by a differential thermostat.

 As far as pre-heating your domestic hot water with this setup, I'd recommend doing a heat exchanger on that, also, which could easily be built. Just remember you want double walled pipe for such an application. You do not want to have the possibility of contamination of your fresh water.

I'd suggest you check your local codes and with  your insurance company.

Local codes here states you can NOT have an open flame heat source in a garage and I'd guess your codes are the same way.

The reason they build "out door" wood burners is to meet code requirements.  Most need to be a min. of 25ft. from any buildings.

From your pics it's hard to tell size, but I think your fire box is a bit small and will require frequent trips out to keep it burning.  

Most outdoor boilers have an "open" water heating tank, to prevent any pressure build up, also requiring to monitor water level.

Besides the insulated pipe that runs between the boiler and heat exchanger you need a worthy pump and their not to cheap.  
Don't forget you need controls.   Controls for the pump and some sort of control for the "burn".   Most outdoor boilers have hi and low temp controls for both the pump and burn (normally an automatic damper).

Not saying it "can't" be done, but the devil is in the details.   It's not just heating water...

Good luck and post lots of pics!

The ATV machine has a lot of surplus energy they add parasite loads like water pumps and electric cooling fans to; the radiator heat exchanger is too restrictive in air flow to demand an existing furnace set up to accommodate it.

The effort required to push X amount of coolant through that radiator for cooling that specific motor was an exact calculation - the potential waste heat counting on a large (100°F+) temperature differential between air and water will allow a small amount of flowing water to carry off a lot of heat without cooling the combustion engine too much to lower its efficiency.

You really don't want to make steam even by accident. Think supplemental heat, not pressurized boiler. Instead of counting on super hot or pressurized water in a small radiator you would do well to go with lower temperature water with a larger flow through a larger radiator surface.

Useful heat exchange from water to air starts at around 40°F differential from ambient; designing a system that can provide a constant flow of liquid heated 50°F above household air temperature to a loop of baseboard radiators (3/4 copper pipe w/ aluminum fins with steel housing) would not be that hard to accomplish. Finding huge ancient cast iron radiators and using them may be even easier.

120 to 150°F heated water drawn from a reservoir like an old water heater, with the circulation pump pulling from the top tank, through the loop and pushing the now cooled liquid back to the reservoir would avoid any chance of steam flashing through the living areas and lower the likelihood of injury to occupants.

Here are a couple of links you might find interesting.

http://www.fieldlines.com/story/2008/1/11/2545/59486
http://www.boomspeed.com/bofh1968/stove/index.html

I'm not sure about the heat exchanger.  

Many of the new wood stoves/inserts have a re-burner, either catalytic, or non-catalytic to reduce the smoke.  

Anyway, if your system has the re-burner, then you will need to capture the heat after the re-burner.

You could, of course, use plain ductwork rather than a heat exchanger.  

Earlier I was also thinking about the difference between the ancient cast iron steam radiators, and the automotive radiators & fans.

 I personally would not put pipe inside the fireplace, but on the outside. Any tank, or pipe system you do install I would have either an expansion tank/pipe on the top (highest position) or some sort of pressure relief on it. Probably both. you will need a considerable amount of surface area on your radiator to transfer the heat to the air.
You also do not want to restrict normal airflow of your current blower or you will decrease the efficiency of the current furnace(if you choose to put this in the hot air ductwork). In other words the area through the radiator must exceed the current duct airflow.

 As far as automation. A differential temperature control switches on a pump(or fan) when the temperature at one sensor exceeds the temperature at the other. Some are designed to accommodate setting this difference, and can also be set to switch on at a given temp at one sensor. If you pumped water to a tank. then you could have the pump  switch on when the fireplace sensor exceeded the holding tank temp. Then water could be pumped from the tank to the radiator(s),(and also switch on the blower fan) when the house thermostat kicked the system on. Also you could put in this tank a loop of copper pipe, acting as a heat exchanger on the cold water inlet of your DHW heater. A tempering valve would also be a good idea in case the temperature becomes scalding in the DHW system. (this mixes hot water output of the tank with cold water to lower the temperature if required)

 Doing it this way would require two pumps, but they wouldn't be running constantly, only as needed. The initial cost would be more, but savings in running the pumps would pay for that rather rapidly.

 If you do it as you drew, you'll have some water trying to rise from the hot water outlet side to the highest point of the tank. (heat naturally rises)
 If you put the cold water into the lowest points, and remove it from the highest, you will get the hottest water out.

 Yes it is fine using the pump on the cold water side.

 I would also install a sight glass on your expansion tank. that way you know when the water level drops. It may even be worthwhile to install a float valve for automatic water replenishment.

 The problem with a tube in pipe heat exchanger is the surface area and the fact you will have to be pumping water when your water is incoming into the water heater.. otherwise you will get no heat exchange. That is one reason (amongst others) I like the idea of a storage tank.

 It would be prudent to look at garygary's system on his house which is very well designed, although it is mainly solar heated.

http://www.fieldlines.com/story/2006/6/6/144733/0442
<img<p> It is a single wall exchanger so may not meet plumbing code but because the inner pipe
(cold water supply to water heater) has no joints the chance of developing a leak
is small and if it did leak the higher pressure in this pipe would dump fresh water into your heating loop and overflow your woodstove tank and not allow antifreeze into your domestic supply.

Check this picture for my idea on your radiator placement, over the black hole in
the return air shoe, not the grill he is pointing at.

src="http://www.otherpower.com/images/scimages/7822/DuctAtFurnace040DFss.JPG" width=80%>

I think your tank on your wood stove would work if you could keep it in close contact to the stove shell for maximum heat transfer.
You would probably be better off with a larger tank, in order to have a cushion, if
you are not drawing heat from your radiator or heat exchanger, the 7 cubic feet of
 water in your tank  (437 lbs.) will heat from 112 degrees to 212 degrees (boiling)
(437X100=43,700 btu) in 1 hour if you had a 87,400 btu fire and a 50% heat transfer
from your fire box to tank.

The reservoir should be at the same level as your tank with a loose cover to prevent
excessive humidity into the shop and allow easy access to check level and fill.
With the reservoir in this position and a large open vent if the water does boil it
won't be blowing a gallon of scalding water out at roof level (think hot shower)

The circulating pump is designed to handle hot water and should be below the tank level to eliminate air locks. If you put it in the return line and any leak develop
it will suck in air and airlock, with it in the supply line any leak will show
immediately and can be dealt with. You may want to insulate the outside of the water
 jacket to prevent overheating the shop.
here's a crude drawing of my idea.