My first question would be:
Have you looked at any of the suppliers who sell with Debian preinstalled?
I would definitely get a laptop/ notebook with an Nvidia graphics card. Nvidia support is excellent. ATI support (even with proprietary drivers) is not so great.
Look at the internal wireless card. It's possible to get a broadcom card working but they don't work well. I've seen laptops with builtin Atheros chipset cards. Atheros, Orinoco, and Prism cards are great. However, I personally prefer wireless pcmcia cards with external antenna connectors su as Ubiquity.