I'd suggest finding one with a decent cooling solution, just in case - some of the cheap laptops overheat even with modest normal use, and that could be inconvenient in a server. On the other hand, servers usually have very light load ...
If you haven't already got a laptop you're planning to use, the thinkpads I've tried have had good cooling (and usually decent FreeBSD support), so if you can find a cheap (perhaps used) one, that might be an idea.