2005 is the only year I would skip...I'll probably get grumped at for saying that. Anyway here is a nice 2006 for under $30,000. I bought his original wheels and I would trust this seller:
For sale - 2006 Boxster S
Myself I'm a 'S' guy, but the newer Boxsters (2005) have 240 hp which is pretty good. My 2000 'S' has 250 hp and it is nice. All depends how much straightline power you want...remember these are roadsters.