I agree that it's very hard (nearly impossible) to regulate the heat in a small greenhouse for seed starting.
Again, depending on what you want to grow, and how much of it, you don't necessarily need much room at all to start some seeds indoors. Even a small (2-foot) fluorescent or LED light fixture would probably let you do a whole flat, and a 4-foot fixture (inexpensive shop light) would let you do 2 flats. Really just a matter of rigging something up to suspend the light from so that it can be positioned just a few inches above the plants. (For initially sprouting the seeds you can just put the flat - or whatever you are using to hold the packs or pots - in a nice warm spot, then move it under the light once your seeds are sprouting.) Alternatively, you might want to consider winter sowing - here are links to a couple of articles on this site that will give you an idea of how to proceed with that:
https://garden.org/ideas/view/...
https://garden.org/ideas/view/...
I hope you're able to give it a try - starting from seed is very rewarding!