Dell are very often used by any organization that needs to order a large set of computers at once, so this is a good bet. They are probably well equipped to supply you with everything you need. In terms of the actual computers, anything that’s mid-range in terms of prices will last at least 2–3 years, especially if you don’t need the computers to do anything particularly demanding. The most important part is to take a look at your budget and see what you can get. Try to get computers that have all the peripherals included to avoid the hassle.