Chris depending on what your keep alot of people vary opinion on flow. Im currently with a 55g, set up as a coral reef type tank, running a MAG 9.5 & two power heads, I consider this to little, and plan to upgrade in the future.

A sump I would imagine it depends what you plan to keep in it. If its a refugium style with space for skimmer and such you'll probably need a wide sump maybe at least 20gallons? Its the space that ends up mattering more than the gallons even though more gallons usually means a more stable system.

Good luck feel free to ask more questions.