Having passed through several laptops and installed for friends over several years, my bias is that there are only 2 Important things: 1) Unless you want to separately buy a PCMCIA card modem, make sure any built-in modem is of a) older controller based type b) the Lucent chipset type which has developed good support under Linux, start at http://linmodems.org for winmodem info.
2) Overall reliability of the brand name, because you PAY a high price for repairs. If you don't need fast graphics, older laptops are fine though heavier. As an inexpensive (not to be mugged for) laptop for a son visiting Russia, I bought an ancient NEC Versa 486 V50 mH, refitted with a 2 Gig drive. After the 12ish floppy Debian install (no CDROM), ppp was established through PCMCIA modem. Apt-get installing all needed including Xwindows was straigth forward after that. Anything newer is bound to be simpler. Very CARELY examine the hinge region for hairline cracks. Hinge cracking is a common laptop problem. MarvS
