I bought my wife a 2005 2.5i X3 last year with 75k miles on it. It is a blast to drive. I second the comment on the air intake boot on the M54 engine. I changed the intake boot after a CEL was thrown. It was a pain to remove all the hoses, but it was pretty easy to do. You may also hear a 'whoosh' while accelerating which could intake a small intake boot tear.
On the staggered set-up: I have a e92 328xi with staggered wheels. The key is to keep the rolling diameter similar between front and rear. My 18"'s are 225/40 F and 255/35 R for a rolling diameter difference of ~0.2%. The tire diameter calculator on the e90post Wheels & Tire section is a good place to do the math automatically. Conventional wisdom is to keep difference less than 1%.
2008 328xi e92/
Black Sapphire Metallic/Saddle Brown/18" 220M Reps w 225/255 Falken 452's
Sways and springs