Hi! Welcome to the forum! It sounds like you're already off to a great start~
Heating: A UTH will work perfectly in a tub. It can be stuck on the bottom (never inside the actual tub- always on the outside!) and controlled with a thermostat. You can set the thermostat to a desired temp and it will stay within a few degrees of your selected temp. This is to make sure that the UTH doesn't overheat and burn your snake. Here's an affordable thermostat if you're on a budget:
https://www.amazon.com/MTPRTC-Contro...art+thermostat
You can get a thermometer/hygrometer with a probe but be sure to not use any tape because the snake can find it and seriously injure itself. So I recommend using a heat gun along with it to be sure that your temps are accurate. Here's one with a probe:
https://www.amazon.com/Samshow-Hygro...ty+gauge+probe
Here's a heat gun:
https://www.amazon.com/Infrared-Ther...words=heat+gun
Feeding: The size of the prey needed is determined based on the ball pythons weight- not age. The prey should be about 10-15% of the snake's weight. Here is a general guide:
https://imgur.com/a/1BARY
You will want to be feeding your snake rats instead of mice because if you keep them on mice it will be hard to switch them over. Feeding mice means more prey per meal. Why feed a snake 5 mice when you can just feed it one medium rat? This is a good supplier of frozen prey and you can choose a smaller amount if you're worried about being stuck with too much of the wrong sized prey:
https://www.laynelabs.com/frozen-rats
Bedding: If humidity is a concern, definitely go with repti-bark or repti-chip. They both hold in humidity pretty well and are aesthetically pleasing in my opinion.
Housing: About 12 quarts for a juvenile and 32 for an adult male. Be sure you get a nice, snug hide that touches them on all sides like a cave. Those half log hides aren't very good for hiding.
Obtaining: Honestly I highly recommend going to a breeder online. MorphMarket has lots of options from many different breeders and Royal Constrictor Designs is a high quality breeder. We also have several breeds are the forum who produce great animals. Always research your breeder! The Fauna BOI (Board of Inquiry) is a great place to look up a breeder and see if they have a good or bad reputation. Most breeders will list what type of prey they are feeding their snakes. I recommend getting one that is already of frozen/thawed (f/t). Most breeders will be happy to give you more information and pictures of the snake. Trust your gut. If it seems shady or too good to be true- it probably is. Keep in mind that shipping will cost about $50. You can get some nice morphs with shipping for $100-$150. Age of the snake doesn't really matter as long as you do your research. Just be aware that younger snakes are more timid and more likely to not eat if they get stressed out. Don't worrying about killing the snake. As long as your husbandry is on point you will be just fine! You're already doing a great job by coming here and asking questions BEFORE getting the snake!
Handling: You didn't mention this but as far as handling goes (especially for a young snake), do not handle them at all for a few weeks after you first get them. Moving to a new environment is stressful for them and being handled adds to the stress. It's best to wait until the snake has had two or three successful meals first. After that give them 48 hours to digest. Start with short handling sessions of about 5-10 minutes. Waiting is hard but you will ideally have this snake for 20 years or more so a few weeks isn't so bad
Let us know if you have more questions. I hope this helped!