Hello gardenmark,
It sounds like you have had a lot of problems. I'm not sure if they are all questions though. Something is wrong if 6 fish tank heaters burned out, there must be a electrical problem. Or they are getting damaged somewhere. Used equipment shouldn't be a problem as long as you clean and sanitize it thoroughly. Scrub it thoroughly with soapy water, then mix some bleach and water (about 1/3 bleach) in a spray bottle, and spray the entire system numerous times. Also soak all the tubing and connectors that you can in a bleach water solution overnight. Soak the
pump in hydrogen peroxide (so bleach wont decay o-rings etc. parts). Clean and sanitize the greenhouse with the bleach-water spray the best you can.
The roots should be able to get air, or they suffocate. I cant see how bad things are, so I don't really know your situation. But I probably would start over. there is a product that might help, but I have never tried it so I don't really know if it works. but here's an option:
Pythoff: Hydroponic nutrient conditioner
Personally I would go with a
drip system for tomato's, probably peas depending on the setup, and a water culture system for the lettuce. But that's just my preferences.