One thing that I would add, or reiterate, is that althugh you can fit X number of machines in a space, remember that you will want some elbow room for when you need to pull one of them apart (ie. shop out, playfield swap or cabinet decals), and that invariably you will probably need room for one or two more on a short term basis (eg. a friends machine on loan, a deal you can't pass up, etc.)

I got lucky with my "external room", and I think that I catch less grief from my wife beacuse of it.

Happy house hunting.