If you're doing it on the cheap, I'd recommend a nichrome coil heater, with the coils arranged in a rectangular spiral, a la Thurston James's plans in his book The Prop Builder's Molding and Casting Handbook, which has plans for 2 x 2 foot and 4 x 4 foot bottom heaters. For a top heater, I'd use aluminum flashing to line the oven, and no millboard. (It's cheap and works very well.) You'll just need some kind of rigid-enough framework to attach the flashing to. It doesn't have to be heavy duty.
You can get 22-gauge nichrome coil from www.infraredheaters.com.
There's a bunch of people who've made Thurston James type machines over at tk560.com. You should sign up on the discussion board over there. It's a good place to ask for advice on making cheap vacuum formers.
Have a look at the surplus Thomas piston pumps Doug is selling over at www.build-stuff.com. If they're the pumps I think they are, I have two, and they're nice pumps.i also would like advice on a vacuum pump to use. I found a 3/4 HP rotary vane pump at Grainger but thats $597!! Any ideas on a cheaper model?