You can't go wrong with a QT15, of any vintage. I have (2) '88 QT15's, (1) '85 QT10N, (1) '85 QT20N, all with the T-2 control. They are fast, accurate, easy-as-pie to program and set up. They just run and run and run.
The most common problem with the machines is blown spindle and servo drives, which are made and repaired by Mitsubishi Electric. Chances are you will get a lot of years out of the machine before losing a drive, but if you do, plan on about 1500 to rebuild the servo drive, 2500 for the spindle drive.
Mechanically the QT15's are rock solid, with powerful servo and spindle motors, and large ballscrews that will easily push any drill you throw at it.
And like Dave said, us Mazakers are here to help.
Catman