This can't be that difficult to answer for yourself, now, can it?

Make a list of all N900 parts you could live without.
Make a list of all N900 parts you could not live without.
See what overlaps. Presto.

Typical items for me to look for (and that have been mentioned in threads around the board only a couple of thousand times...):

USB port. Does it wiggle? Don't buy.
The screen. Lots of damage? Don't buy or replace the screen.
Do all the keys of the HW keyboard work. Does the KB slider wiggle?
Was the device overclocked? Above 900 Mhz? Don't buy.
Bring a SIM card to check if it works.
Run fsck -a to check for errors on the filesystem.

Etc.