Hi,
two additions:
If you buy one with a DVD reader and no cd-rw, check a couple of DVDs. My drive on the Ti500 (the same as in earlier 550 Models) broke twice. It didn't recognize and mount dvds after a time and cracking sounds could be heard while ejectiong the dvds.
I think the first slot loading drives have been mechanically poor constructed.
A good thing would be to go for a TiBook with combo drive.
Check if the display can be opened smooth- and easily. The hinges are very tight in the TiBook and if they are not oiled from time to time it gets very hard to open the display. Result can be case breaks around the covers of the hinges (or beneath).
So it won't be a bad idea to remove the covers (torx screwdriver required) and see how it looks like.
All this did not only happen to mine, it is well reported in many forums (including this one).
Good luck,
Michael