Speaking from experience, keep away from stairs.  I've got WAY to many of them and its a pain in the ar ^.^e

+1 with Marty's opinion. A lot of the guys have seen my old place and know the stairs both inside or out woul dhave put even the most avid collector off pinballs.

I'm building as we speak (yes its started guys  ) and its two storey but the games room is on the ground floor and the largest step I ever need to make is 60mm from the garage. What you can do is design a sizeable room and make sure you sound proof it and put a decent door on it. Also minimal windows for max wall space for hanging stuff and pinball space. That's what I'm trying but can't tell you how successful it was for at least another 11 months.

DON'T HAVE STAIRS unless your wife has a strong back