My advice would be to go as big as your stand and or wallet allow you to. You can never have to much water to keep the quality up. Measure your space in the stand and compare it ro
a standard tank dimentions list to find the best sizes then wait till one of those tanks are being sold used and buy it n build it.
If you find a good sump for a good price its not a bad idea to use it for now just make sure you can get your money back on the resale if you decide to upgrade later. Use rubber hoses instead of pvc
so you wont have to re plumb the whole thing and cost you even more time and money.