Funny, I was about to suggest a 600X

. I know, a shock huh?
The reasons
for the 600X (when compared to a T23) are:
- cheaper; at least for the basic system (notwithstanding the cost of PC100 memory but some eBay bargains can be had)
- sturdier; certainly more so than many of the T23s I've seen (but then again, I mostly buy problem systems

)
- one of the best keyboards that IBM offered on a Thinkpad (personal opinion)
The reasons against the 600X, and in the T23's favor:
- bigger screen (13.3" vs. 14.1")
- availability of SXGA+ (1400 x 1050) on the T23 (600X is XGA; 1024 x 768)
- usable with the 2878 dock
- faster processors (T23 minimum is 866Mhz, max is 1.2Ghz; stock 600X is 650Mhz; mods can go to 850Mhz)
- more maximum memory (T23 can go to 1024MB; 600X limited to 576MB)
- no Thinklight, built in ethernet port or wireless available on the 600X
Of course most of these things are because the T23 is usually more expensive than the 600X.
If I had to consider which one of these systems is more
reliable than the other, and assuming that we are talking about systems which have not had any professional refurbishing, I'd have to go with the 600X. I know of several weak points on the T23, which if you have the time and capability, can be fixed. However, I am the Information Technology and Field Service departments in my house and have the tools to fix most problems. I would assume that the OP daughter's main goal is to have something that will work day-in and day-out for the next several years and not have to worry about visits to "the shop".
My younger daughter (16) has been using a 600X practically non-stop for the past three years. It's on almost 24/7, gets carried around the house every day, used on the couch, in bed, in the bathroom (don't ask) and has not had any problems at all. It's a 500Mhz system and has full memory with Windows XP. She mostly uses Firefox, Word, AIM, Audacity, Paint Shop Pro and iTunes on it. Of course a T23 could run those programs much faster at the slight increase in overall size and probably at a higher temperature but she's happy with her system.