The tower version is hard to come by, but if you look on ebay.com now I see several good deals on the desktop version. You can then get an elbox mirage tower case and be set if you decide to get a Mediator as well for some real nice upgrade options in the future.
Or if its just the Amiga look and feel your after you can get a Pegasos II and run MorphOS like I do. its a very nice Amiga close that runs real Amiga software with a 1GHZ G4 PPC in it. Its a lot less money to get it up and going then a real power Amiga these days, and its a lot faster.