I'd say forget the pre-built nanos. A lot of them need to be modified anyway, which is a waste of money, IMO.
You could buy a 20-25 gallon tank for pretty cheap, drill it, and add a sump/'fuge using another tank (like a 5 gallon).
I built my own "cubes" and they turned out pretty good. I'm going to drill my smaller cube in two places so I can get rid of the overflow tube and the powerhead hose. mops.ca sells decent glass drill bits for pretty cheap, and they work.
Here are some pics. They're a bit dated, so my tanks look a bit different now. (Don't bother with the Lee's skimmer or the Current USA Nano skimmer.)
13.5 gallon display w/ 4 gallon sump:
6 gallon display w/ 2 gallon sump:
Pieces I used instead of drilling the tanks:
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