Hi and welcome to the forums. There isn't much that is required for these little guys, most of the items are just quality of life and the negative effects can be mitigated if you're willing to put in the work.
As mentioned, the tank size is up to you, but the smaller you go, the more frequent the maintenance. We're talking daily or every other day for 1 gallon with weekly or bi-weekly for 5 and 10 gallon (again, all depends on the setup and parameters).
- A heater is required unless you live in a tropical climate and can guarantee stable water temperatures around the 80 degree F range.
- A good water conditioner is required for tap water, and highly recommended for all other water sources.
- A filter is not required but very nice to have as it helps with water parameters between water changes.
- Decor is another nice to have and at minimum you're aiming to provide him with a place to hide from predators outside and above the tank.
You could probably get a 5 gallon on sale for about $15 or a 10 gallon at the PetCo dollar per gallon sale for $10. I'm not sure when the next sale would be but it should be soon.
I don't know how much power you have to play with, but with a larger tank, you probably could forego the filter. You would need a bigger heater which would be a tradeoff, but could always try to insulate his tank.