Building a new tank need opinions

[CHEAPREEF]

Well i've decided to build a tank to put both my nano reefs in, should have done this in the first place rather than building another nano. The tank will be 24x18x18, around 30gal. The overflow and lighting are the only things i'm not positve on.

The overflow i was thinking of building on the outside back wall in the center, so to not take up so much space. Any pros and cons on this? The second option was a setup like Stircrazy has with a single pipe center back. Anybody have any other suggestions, i would like to get around 700-900 gal/hr flow.

Now for lights, i want to use 175watt MH. I have two ballasts and sockets, but only one 10,000k bulb right now. Would one be ok over this size of tank or will i really beifite from two. Either way i will be supplementing with 3 actinics overdriven by a workhorse 7. The water column will be 14-15" with the DSB, and i would like to keep some SPS and a few clams.(already have one.)
Thanks for any advise.

Clinton