If you can get your hands on a good ThinkPad T42 you'll have a very nice little machine. ThinkPads are renowned for their build quality.
You'll find the hard drive capacity a little limiting by current standards, and unless the RAM has been upgraded you'll need to reserve some budget for doing it - the machine only had 512Mb as standard.
The HP NC6000 was a great laptop when it was first launched, but you should look at the installed RAM - 512Mb as standard isn't enough.
Of the three, and all things being equal my personal choice would be the Lenovo X40. It was a superb machine in its day - I had one myself- and one in good condition is worth snapping up.