For seed starting & transplanting, you could even just build a few boxes... say about 2 - 3 feet deep.. solid sides & bottom with glass windows on top. Insulate around the outside with hay/straw, mulch, leaves, sawdust.. anything warm and comfy! Make sure you put hinges on the windows though so that you can get air in there when it's not too cold out!
Also, I have built structures out of that sturdy clear plastic & PVC pipes & wood (for turkeys and broiler chickens, and vegetables)... You take wood boards and build a frame as the base, then arch the PVC pipes, using the wood to hold it in the arch shape. Then take the plastic and stretch it tightly over the entire thing. For doors, I would recommend having 2 for adequate air flow (on the warmer days). Those could easily be made out of wood, glass, and/or plastic. My vegetable structure was a 12 x 12 ft structure with 20 foot PVC pipes. A nice decent size! You can grow a lot in that!
For insulating it though, you won't need to.... but the only thing is that depending on where you are (where are you?) the ground may already be frozen where you want to set it up. This type of structure is good to set up and get going before the ground freezes so that the solar passive structure can be heated by just the sun & keep the soil warm enough to begin the early stages of those precious little seeds.. Or double up on the plastic!
So as a suggestion, I would say to go ahead and direct seed into very nice soil/humus mix in those boxes I was talking about, and then transplant into the greenhouse once it warms up a bit.
And try going to any conventional nurseries that have greenhouses! Those places have TONS of extra plastic laying around that they ship off to china to be made into those shitty toys you buy at dollar stores... I used to work at a nursery & there was more plastic than we knew what to do with.. my boss was grateful whenever I took some!
Those two little structures have done me good! So good luck! Let me know if you need any more information about anything agriculture related! Or if you need seeds! I worked for an organic seed company up here in VT and was able to hoard a bunch! =)