I've always used plastic, but for the small 16X6X7 house I'm building now, I've bought fiberglass. It will cost about 350 dollars, but it will be much better looking, and something neat and tidy does seem to set a standard when one works.
Where I'm at, we seldom get more than a light frost, so by closing a well constructed greenhouse at night I don't see the need for a heater, and by putting windows in the right places I've even grown flowers in the Kalahari desert without a fan.
In the drier areas of the US many people have their gardens and grass on an automatic timer, and most of these models can be set for several times a day. So, with maybe 50 dollar in parts, your watering system can be completely automated. I put these systems in for a living, so please ask if anyone has any questions about the wiring.
A small hint for windows: piano hinges.
So, I agree that for 7 or 8 hundred dollars you can make something to be proud of.