Okay
GOD!
I had this whole explanation written up on the sump and the stupid browser closed on me. Piece of crap.
Anyway, here it is again:
I've narrowed the sump to 12" in order to get the Snapper in behind the whole thing (or the front if you want ease of maintainance). I've also plopped in a few upgraded equipments just for show. Aside from that, at 18" tall, you should be looking at a 25 gallon sump. If you're concerned about an overflow (power outtage), just make sure you plumb the return into a manifold that sits on top of the tank or suspended under the frame. This should reduce, if not remove, any drainage into the sump.
The last thing you'll need is a good auto top-off unit (PM me) and you'll be set.