I have a small greenhouse, built from an old shed and a raft of used windows. Roughly 7' x 8'. With it, I start my plants in the house, then start up the greenhouse on the 1st of april, which is 7 weeks before we usually set out bedding plants. a 1.5 Kw heater gets me through the cold nights.
This is a good way to start -- see if it's fun.
I want to build two more: One will be a 'conservatory' attached to the house. Used as a sun room, and a place to start seedlings. I also want a 30x60 foot gothic hoop house for production use -- forcing lilies for the spring trade
If you are in a cold climate (We're zone 3) then running a greenhouse in winter isn't reasonable without a bunch of supplemental heat. If it is well made running it for 8 months a year can be done.
Consider building a greenhouse attached to the south side of your house. Even if you shut it down for the winter it can provide heat to the house during sunny days.
Build as much water storage as you can into a green house. This reduces temperature swings.
Your aisles should be wide enough to move a wheel barrow down them and squeek by it.
A good cheap way to make benches is a double row of barrels (filled with water) and used pallets. Note that the cracks in the pallets make wonderful places for pests to overwinter. There is merit in closing the greenhouse up and get the temps as high as possible for a few days at some point each year. (140F will kill many eggs)
Reaching over 2 feet into a row is hard. Wall benches should be 2 feet wide, center benches can be 4 feet wide. With 3 foot aisles this makes for certain magic numbers for width:
* Two wall benches + 1 aisle = 7 feet.
* Two wall benches, 1 center bench + 2 aisles = 14 feet
* Two wall benches + N center benches + N-1 aisles = (N+1)*4 + (N-1)*3.
Watch kijiji and craigslist. Sometimes you can get a used greenhouse frame for cheap.
The usual problem with a greenhouse is NOT heating it, but cooling it. In summer you need an air exchange a MINUTE to keep from cooking it.
This can be done with high speed fans, or roll up sides, or complicated roof mechanisms. In the southern US they often remove the plastic for the summer.
In your research google "high tunnel" Cornell has a good set of resources for it.
Building an attached shed to the green house is a good idea. This is where you do your potting, keep the watering hose, all those bits. In the trade it's referred to as a head house, as it was usually on the north end of a bunch of N/S running greenhouses.
If you want an interesting take on low energy cold climate greenhouses google solaroof (Note only 1 r) This is a technique using soap bubbles as insulation/sun shade between two layers of plastic.
If the greenhouse is seasonal then being able to separate it into chunks has merit. In early spring when starting seeds, you only need a small chunk at one end that is kept warm. As plants are moved to larger and larger pots, the partition is moved down. In this way a moderate heater can heat a small area during very cold times, and a larger area in not so cold times.
On a sunny day, a greenhouse needs an air change every 10 minutes just to provide CO2 for the plants. This varies with the amount of green in the house. In winter you want makeup air to be well mixed with the existing air before it touches a leaf. In commercial houses this is done with a 2' diameter poly plastic tube with small (about 1 cm) holes every few feet. The tube is hung from the ceiling.
If you are clever, you can reduce outside air by using a compost heap for your CO2 source. You have to watch the C:N ratio as too much N and the heap will produce ammonia gas which is tough on your plants.
I've also heard of using chickens as your CO2 source. Chicken crap has a big ammonia problem.
If you get a used greenhouse, take a LOT pictures of it. Bring a pad of paper and make signs: "Top end connector" "Mid wall perlon"
As you dissasemble, take another set of pictures of the piece, and the bolts. Not all the bolts are the same. Use up the 2.25" ones where the 2" ones should have gone will leave you short later. It's a good idea to put the bolts back into the connectors.
Do some careful measuring. Some greenhouses are sent with all holes in place, some you have to drill on site, some are a mix. If the holes are drilled on site, they will vary in position. E.g. if one rib is an inch out of line, it's holes will be an inch out of line. This will make reassembly tricky.
If the holes are predrilled, you move the pieces to meet. If they are site drilled, you can mark every piece with location and hope, or you can resign yourself to drilling new holes. Drilling new holes isn't usually a big issue.
If you live in a snowy climate get a gothic arch. Round tops collect snow.
Do your homework: Research the brand, and check the rated snow load. Snow will usually slide off a gothic arch roof in a few sunny days. If you ever get 2 feet of wet snow at once, you need a roof that will support that. A cheap greenhouse will have a 10-15 lbs/sq foot load capacity. Good ones run about 40-50 -- same as a house. You can compensate in part for this by putting the ribs on closer spacing.
Larger greenhouses are easier to control for temperature. So are taller ones.