1. 20 gal tank could last it's entire life for a male. 40 gal breeder tank(36x18x16) would be big enough for an adult female. I wouldn't start a baby or juvenile in any thing bigger than the 20 gal.
2. For only one pet snake I'd go with a reostat and save a bunch of cash. Use a thermometer with a probe on your under tank heater to adjust and moniter the reostat setting.
3. Zoomed UTH is fine.
4. Scratch / don't need or want this. It will cause your enclosure to have no humidity that your snake needs.
5. Aspen is fine and looks good. Get a bunch it will have to be spot cleaned regularly and changed out monthly.
6. Get hides for the size of your snake. You could just crumple up paper(black or brown construction paper doesn't look bad) for the snake to hide under. I've also used butter bowls, etc. Turn over and cut a cave like entrance.
7. Ball pythons don't need the climbing log.
8. Works for me.
9. Temp guns are expensive. I use thermometers with probes.
10. You can't kill mites if you don't have them. A clean enclosure should stay mite free.
11. Common cleaners are fine. 409, etc A disinfectant cleaner would be more desireable. If you start a collection of snakes, use a good disinfectant cleaner like Nolvasan. One snake won't catch a disease from itself. They are clean healthy animals if you give them what they require and keep their enclosure clean.
12. Screen tank tops: Cover up to 90% of the top with plastic, aluminum foil, etc. to keep in heat and humidity.
13. Have fun!