It might take a while to find the right deal, but I bought used with only about 115 hours. In my case it was a B2620 with a mmm and loader. I have recently added a BH6500 backhoe that was never used by the original owner.
I see others once in a while for sale on Craigslist. When you're ready, try using SearchTempest to expand the cities to search in.
People buy stuff and then change their minds to buy bigger, smaller or just get rid of it. In my case, I saved a lot of money.
As to size, I would think a BX or similar size in other brands would be fine for what you described.
As far as I know, for the pool, you pull the liner and demo the sides (maybe galvanized metal panels about 3 feet tall all around), then back fill. I helped my FIL put in an inground pool 30 years ago, The sides were as described and the bottom and tapered sides were sand screeded and worked like concrete. Then a plastic liner covered everything. A mechanical or hydraulic thumb would come in handy for the demo.
By the way, nice job on the water management on the current house.
Used can definitely be a money saver, and good used machines are out there. I did not mean a total condemnation of the used market, I could not find the right machine, equipped as I wanted at the right price. The closest I came was around 12k for a used when the new one cost me 16.5k. Happy land and tractor hunting, you will not be disappointed owning a tractor.