whisper, imho second hand laptop might not be a good plan. Here are, in my experience, of me and colleagues most common laptop faults:
(1) Battery stops charging. As with all batteries they have a finite life. Depending on the brand, size of battery etc expect to pay £50-100 for an aftermarket battery. Possibly more for something 'clever or fancy'.
(2) The chargers fail. Not necessarily a big problem, as MAPLIN etc will sell generic chargers for £50 ish.
(3) The screen fails - on a laptop this is usually game over. Whilst it might be worth replacing on a high end machine it won't be on an old one.
(4) Hard disks fail. ("Old school" hard disks have spinning parts which don't like being thrown around whilst moving. Newer, esp high end, laptops will have 'solid state drives' that are no only more robust but faster.) A new hard disk will likely cost £50-100; and then you have the fun of restoring the backup, reinstalling everything etc.