I think having the overflows is still nice option even though you dont want to run a sump... they are a better option IMO and I really dont understand why some people over think the complexity of them, They are easier to maintain then a canister.
Anyways, you could always plumb your overflow to connect with your fx5, put your heater in the overflow box to give your tank a clean look. I did this about ten years ago with a 135gal and 2 xp3's. I run a sump setup now and couldn't imagine running a canister again... hated opening it up to clean.
Either way I am sure the fx5 is a great filter but If it were me I could run a sump on that tank since it already has an overflow.
good luck and keep us updated on the tank build