if u dont want any power heads u should do a closed loop system ...
as far as ur sump return goes u really cant add a bigger pump than what ur overflow box can take in id say the mag 9.5 is all u need on that anymore it would empty ur sump out u can probably use the mag 12 for the return but u may need to put a ball valve to throttle it down somewhat if it puts out too much water
if u dont have a closed loop system ur probably guna have to add some power heads to get more flow in your tank
what do u plan on keeping??
its always best to get what u want the first time to save money and headache later on....
ie ... save up for the right lighting, skimmer, circulation equipment, and other helpful equipment