Can I assume that the reason you are building the greenhouse to to keep the bugs off the tomatoes? WHen I think of green house i think of a structure covered in plastic.
Have you considered growing the tomatoes using a row cover to keep the bugs off.
The green house (covered in plastic ) will need to have vents to keep it cool in summer. It will get very hot, depending where it is located.
Yes 5 gal, food grade buckets will work, Look for instructions on how to make self watering pots - on internet.
Maybe make a raised bed, use conduit to form hoops making a cold frame structure and cover with a screen or row cover or something that allows the air to flow.

I didn't consider row covers. I looked all over online for something similar to bird netting but with smaller holes to keep the bugs off but I never found anything that would work. All I could find was screen type stuff that would keep bugs ff but would also provide too much shade considering it would need to stay on 24/7. A greenhouse seemed like a better solution. I will put a screen door on the greenhouse in addition to the two 8ft windows (which will remain open) and if necessary I'll add a fan to keep it cool. I will look into the self watering pots. Thanks for the suggestions.