I got a Thinkpad X40 I will be putting Windows ME on. It will likely have nearly the same hardware as the T42 and similar Pentium M Thinkpads. The only downside is the X40 seems to only ship with Intel Extreme Graphics 2. Yeah, it's not a great video chipset by any stretch, but it should work for my late 90s / early 2000s purposes just fine. Besides, even a 1.2GHz Pentium M is still pretty fast for Win9x. The main draw here is a 12" Win9x laptop.
Although yes one could put WinXP on any decent Pentium M machine, I find it a waste of Win9x support which is hard to find in laptops. Usually, if you find a machine with a Pentium M (and therefore an Intel 845 or 855 mobo chipset), odds are good it'll have either an Intel, Nvidia or ATi video chipset with Win9x drivers. IBM often used SoundMAX for their Thinkpads, which has Win9x support. Dell usually has Sigmatel audio, again it works with Win9x. The only tricky part of any recent laptop and Win9x is getting Ethernet/Wi-Fi drivers. They are hard to find, but do exist.