I know a handful of people who own stand-alone prototype consoles (blue/grey) as well as final units (black). Those can be added, as well as the dev cards and the drive box, the prototype controllers (yes, plural), maybe the two kiosk systems, and of course the arcade games. And prototype games do exist as well, I own two myself. So there's plenty to add. And since I'm collecting M2 stuff for three years now, I can pretty much estimate the prices and rareness of most of the stuff.