Some cars built before Jan.2004 were known to have syncronizer problems with the 6 speed that would cause them to pop out of gear. I believe there was a TSB on the problem, but I would think by now most cars would have been repaired. Build date is on the drivers door.
If you're considering a 6 speed, I would take the car for a long test drive (in heavy traffic) and see if it pops out of gear. If it doesn't happen in 20 or 30 miles, then you should be OK.
Check for door rust under the rubber seal that runs along the bottom of the door on any year Crossfire. Other than that, I'd just look for the lowest mileage car, in the nicest condition for the best price. Carfax is a good idea, but not always 100% accurate.
Good Luck