I would go for two Minis - used or new - because a) they are somekind of low level but on the other side provide enough performance to not get on your nerves, b) occupy little space, c) can run this or the next version of OSX pretty well, d) are sure worth the price as i guess you'll be able to get the money out from the games as mac users tend to pay for stuff.
If all goes wrong you at least could test multiplayer games on your own... ;O)
If all goes wrong you at least could test multiplayer games on your own... ;O)