One of the most important things is how densely stocked the tank is going to be. It it is going to look like a pet shop or an Asian tank, you will need a fairly large sump. The general rule of thumb is 30% of the tank, and your worries will be solved (Asian stocking needs 50% and larger).
Off the shelf sumps are sized way too small for my comfort. When the manufacturers start competing against each other, they tweak the numbers to be in their favor (not in your best interest or your fish's).