Southern car (aka no rust)
Any collision damage? (check VIN)
solid maintenance history - or you know the owner and how they maintain their vehicles
A/C that works properly (expensive fix)
check all the electrical (switches, seats, windows, sunroof) - though most of this stuff is pretty easy fix
starts well (all glowplugs working)
shifts well (pretty subjective - but firm)
Target sub 200K (so the timing chain should still be fine)

__________________
~shell
As of 2/2010:
2001 CLK55 0o\=*=/o0
13.6 @ 106mph
10K mi
1984 300SD
260K mi and going and going...
97 S600
46K miles
1991 Sentra SE-R (extremely dorked with)
www.se-r.net