You can use 2 layers of greenhouse plastic and blow it up with a bathroom fan. Ive done it more then once. If your going to grow all winter think about insulating the floor.You can even use lights to add hours to the day.As far as heat goes I have used a vent less heater with good results ,Natural gas is your best bet if you have it and I would recommend a vented heater.

I would say NG or even propane would be a better/less expensive alternative to electric. If you dont have access to Natural Gas, then the local propane dealer should be able to hook you up with one or two larger portable tanks like they use on motor homes or maybe even bigger. Any insulating that you can do would be a great help. I wouldn't worry about the moisture in the air. Most peppers come from relatively humid climates.
Good luck. A green house that big would sure be nice to get the season started early.
Bruce

Google up "Hot Dawg" heaters. I've had a 36K BTU unit for three winters now, and it works great. Hang it as high as you can, and use some kind of fan system to keep the air moving around inside of the structure. The fan on the unit will blow hot air out to 25'. It also has an exhaust fan so you can run a horizontal chimney pipe. Whatever you do, be sure and ground your gas line to prevent lightning ground pulses.
Mine is coupled to a 150 gallon propane tank.
I keep my GH at 52 degrees, and in a cold Texas winter like last year, with many nights and days below freezing, I spent about $300 on propane, from mid-November to late March, or early April.
The unit is 75 pounds and is a snap to install and hook up.
Here's a pic.
PM me if you want more info.

Gas, AC, and flue detail. It needs a thermostat too, which you see at the bottom left hand corner.

Thanks for the info guys.
pepperdave, my floor is concrete, not sure how to insulate it, I'd assume it would give off geothermal and solar heat, not as much as the earth since it's white and not dark.

We have natural gas, but to run the line to the greenhouse would be a few thousand dollars, from what I've been told. I've already talked to the local propane company and they've given me costs, not bad.

Bruce, but peppers are from warm/humid or warm/dry climates, not cool/cold humid. I think it's the cold wet that causes the issues, like early blight.

Thanks Tom, I had forgotten about your greenhouse set up, you have got a really really nice setup. You wanna come up and help us re-design our greenhouse and set up the heating? ;-)
Pam

I didn't know ya'll had a concrete floor. That's a great heat-sink. Way uptown.
I can work per-deium remotely for adequate compensation, with beer-breaks...