At my uni department we use laptops for everything from programming to sculpting high-res meshes in Zbrush andmudbox, and with 50 peole using laptops of all makes(from alienware to acer) and sizes(a freshman recently bought onee of those giant 20" models) I have a lot of experience. Most laptops will start to break down after 6 months, after a year there will be guaranteed faults and after 2 years they should be replaced. Exceptions exist but only confirms the rule. My 18month old Acer 5024 has broken down once already and the screen is about to come off, just from normal daily use.
Never ever buy a used laptop unless the warranty is still valid, you will just end up paying so much for repair and service that you could have bought a shiny new one for that money instead.