Dell, gateway, and toshiba come to mind. I would buy one that parts are plentiful and can be easily purchased. Dells have tons of parts I know for sure like on ebay. Some AMD laptops have problems with heat unfortunately but most are ok. Some video is upgradeable but not much. Hard drives, ram, and the drives can be swapped out for most laptops. What are you going to be using the laptop for? You should be concerned about weight if you plan on taking it to different places. What about battery power, every gonna need it where you can't plug it in? Do you need a burner in it or a dvdrom?
These questions will get you started.