Personally, if it were me, I'd go with at least the 9000 and possibly with a 12000. They can be cut back to 10% of their output volume so why not to with the biggest one.
The dc pumps work great so far I really do like them but they don't have much had pressure, I would go with the 9000 cause you will have the power available if you need it and don't have to run it 100% percent if you don't need the power.
Lots of room for add on's.
I've got a 9000 running on a single overflow 75, I run it on the lowest setting and it's plenty. You don't really need a lot of flow through your sump but the extra gph would be handy if you want to run some reactors off the return.