Actually, most computers will work fine with Trisquel. The big problems are
AMD video cards and integrated graphics, which don't support hardware
acceleration; new Nvidia cards, which might be crappy or not have support at
all (support for them comes from reverse-engineering, which takes time); and
a lot of wireless cards. For the wireless cards, you have a decent chance of
getting something that works if you only get computers that have Atheros
wireless cards with only Wi-Fi, no Bluetooth. Other problems would be only
annoying at best, such as not being able to use some of the "Fn"-mode keys or
not being able to disable the touchpad.
That said, the best choice would be to get a computer from Think Penguin (but
go there through [0]; that makes the default OS Trisquel instead of Ubuntu,
and it also makes some of the profits from purchases go to Trisquel).
[0] http://libre.thinkpenguin.com
