1. yes, a 20 gallon will work for a 3 footer

2. no, but get a t-stat or atleast a dimer for it

3. look at 2.

4. yes, but dont get any 'kits' they usually have a whole bunch of junk in it that you dont need. all you need is 2 hides that will fit snug, a water bowl, and a heat source (the heat mat) they might not have dimmers or t-stats but you can get dimmers at Lowes. you wont need to buy any substrate because you can just use newspaper.