I've heard time and time again that Ford/Chevy diesels will go around 200k miles before needing a rebuild and Cummins will go 300-500k before needing a rebuild.
Of course there's exceptions to every rule, I know somebody's gonna say "Oh I got 837,943 miles out of my powerstroke before it died" or something, but I think it's the general rule of thumb.
That said, I didn't flinch once when I bought my '04 with 168k miles already on it.
__________________
-- Taylor -- 2004 3/4 ton, 6 speed, 4x4, 192k miles Silencer ring missing, straight piped, AMSOIL 4510, Clearance lights, Mickey Thompson 325/60/18 Baja ATZ's, 2" leveling kit, Predator Stage III