Get a P3. P2s are fine, but the P3 with the 100mhz bus vs. the older 66 really makes a huge difference.
My main laptop is an IBM Thinkpad 600x. It is a P3 450, with 320MB RAM, and a 6gig HDD, USB, 2 PCMCIA Type 2 slots, and a great XGA 1024*768 13.3" screen. Gets about 2 hours of battery life (or about 3 hours with a new one), and accepts dual batteries.
Dunno how much you wanna spend. I will be selling this machine coming next week or so, because I need a faster system. It is worth about $450-500 or so in terms of eBay prices. You can still get loads of parts for them too. The 600x is a very sturdy machine, just solid.
Anyway, enough about that. When you do look for a system, make sure you keep a few things in mind. If battery life is important, look for a system that batteries are commonly available for. Many older systems have impossible to find or no longer made batteries. Also, keep an eye out for proprietary stuff. Many older laptops used proprietary RAM which is VERY expensive and again hard to find.
Hope that helps