I would personally stay away from the 2007 models with an automatic gearbox. The 6-speed AT was new for 2007 and has a tendency for significant software and hesistation problems that often cannot be fixed yet.

Going to a BMW dealer and purchasing a CPO is more expensive, but it does offer some peace-of-mind since there is that extended CPO warranty that isn't offered at other dealers. The X3 has held onto its value fairly well, and thus, prices aren't going to be as low as say, a Chevy Tahoe. A 2006 X3 in good condition should run in the mid $20s depending on options and packages. I strongly suggest finding one with the xenon headlamps (easily discernable from the projector headlamp bumb).

Of course, if you can drive a manual, that's the absolute best way to go for any year. A 2007 X3 with 6MT is an absolute blast.