just my 2 cents worth...
mkay, first off, I would avoid the use of large external high watt pumps for flow, let alone for combined duty of return and closed loop(glad you decided against that), or chiller/CArx feed. as mentioned if it fails, everything is toast...
And obviously this could be fixed by having a spare pump on hand to switch out...But for the cost of those 2 pumps, all the plumbing, gate valves, hours out of your life, you could probably have a dual tunze/vortec set up w/controller.
which would be vastly more appropriate TYPE of flow for your corals and fish,
consume WAAAY less watts. Also, there's the maintenance factor... all plumbing/bulkheads should probably be replaced after 5 years, (unless it's all sched 80, @ doulble the cost) and it should be designed with unions and valves as gary mentioned for removal and maintenance once or twice a year...UGGGGHH!! I dont know about you, but I'd rather just drop a powerhead into some muratic acid....
So..If it was me...I'd run a eheim 1262 for a return pump, and If I did anything at all to the oveflow box, it would be to hid a tunze in it...
and i love the new large tunze that look like the nano's:
http://www.premiumaquatics.com/Merc...duct_Code=T6125.000&Category_Code=Tunze_pumps