The way that I use it, its just producing domestic hot water (showers etc.), but the collector area and tank size could be expanded to do what you want to do.

Its a very simple and inexpensive system. You could build the collectors as shown, or, if you have a good source of used collectors you could use them.

It seems like the large and non-pressurized tank would work out well for your needs.  For the shower water, you could use the large PEX coil heat exchanger.  For the radiant slab floor (which I really hope is well insulated underneath!) you could either draw hot water directly out of the storage tank and circulate it through the floor loops, or use a copper pipe coil heat exchanger.  If you want to collect heat from other heat sources (like a wood heater), you can pick up heat from a coil in the firebox and circulate this heated water through a pipe coil heat exchanger in the storage tank (just be careful to include a pres/temp relief).
While the tank construction may not look like a shiny stainless steel tank, these box style, EPDM lined tanks have a very good track record.  I have heard from people who are just getting around to replacing the first EPDM liner for a 1980's tank!
They are cheap to build in large sizes and you will want a pretty large one.  For what you want to do, you will also want a lot of collector area.

Its unlikely that unless your building and slab are very very well insulated that it will be practical to heat entirely with solar, so you will need backup heating sources, and using this large central, non-pressurized tank makes it easy to add heat from other heat sources to the common storage tank.

If you make the collectors like the two designs I show, I recommend that you use the copper tube, aluminum fin design rather than the PEX-AL-PEX, aluminum fin design.  The copper is more forgiving of high temperatures that might come from stagnated collectors, and with the price drop in copper, and the better performance of copper, there is not a lot of money to be saved using PEX.

Between the collectors and the rest of the system, there have grown to be a lot of pages, and its getting a little hard to wade through -- this page gives a roadmap of all the pages, and makes it easier to not miss something important:
http://www.builditsolar.com/Experimental/PEXColDHW/RoadMap.htm

Don't know if you have thought about this, but the project may qualify for the 30% solar tax credit.  If so, you would want to get those used collectors if they are SRCC certified -- the certification is "required" for the tax credit.

I did a remodel on a 10 year old wood fired sauna. Problems were>

1. Original structure had no stub (stem) walls on top of Concrete slab floor to keep wood above floor level. Everything had rot because a Sauna has huge amounts of condensate inside.... even more with with a wood fired.

2. The stove access door was located inside of sauna.... thus ashes, wood, had to be carried through the entry door and made a huge mess. We converted to a unit with firebox access on outside of building.

3. Had to add external combustion air port through the masonry wall because whomever build it forgot to do so.

4. Interior surface has to be water proof on an outside free standing installation in order to keep the walls from absorbing way too much condensate..... unless you plan to keep it fired all winter long. The mold that grows deep in the walls is really hard to get rid of.

We later added a vent in the ceiling for fair weather times to "Air Out" the inside in good weather.

And finally.... the Electrics/Controls.... Make sure to buy the absolute best damper, thermostat and air control you can get. Make sure it has a safety "fail safe" for "Over-Temp". Wood burning is very hard to regulate and the inside temp can be deadly if not closely regulated. It is no longer legal in my state to use wood fuel for a sauna or steam bath. Also..... though likely not relevant, they are not allowed on many Homeowner Policy's.

As an extra safety device I added an audible alarm for 130 degrees F (Wet Sauna) just in case all else failed and the owners fell asleep....

Northern climates the slab edges must have a foundation extending below frost line.
http://home.howstuffworks.com/home-improvement/repair/house4.htm
Insulate outside of this foundation, cost less by using less... maybe. (depends how deep)
Since you must dig a foundation, why not put the tank below the changing room?
Save space. Tank must be insulated, triple use the foundation/tank/insulation into one!
Benefits: Tank warms area above, more floor space above.

Make the solar heat collector as GaryGary suggests into the roof, not separate saving money.

Sister wanted something like yours, called it a bath house.
The Sana was convertible into a jacuzzi plus a windowed view (special door).
The Sana floor is the jacuzzi cover.

Housing market crashed before they sold, did not get to build it.