What is the shutter count? I bought a 1diin over the summer, and I love it. Rock solid, amazing AF (with 200mm f2.8L), good up to iso 1600. Don't expect to crop much, but stuff looks great online, I haven't had the chance to print yet, but if you look on fredmiranda or photography on the net there are threads just about these cameras that are over 100 pages, lots of tips and examples. Only downsides are that it can take a while for the buffer to clear if you do bursts. It is something like 22 raw, 45 jpeg, perhaps slightly less on the 1dii.
I would say check the shutter count, you can generally expect 250k, but they can last longer. I don't think canon services them anymore though, so you may need to look on eBay and find a local shop (or dit). I bought mine with 35k on the new shutter, so this is all I would really be concerned about.
One last thing, make sure you have some nice lenses to take advantage of the AF. F2.8 or better L lenses focus insanely fast.
treadmills are the trick to everything. fat chicks, skiing on grassy flat back yards, OK GO videos.. - jibnut