Even if your needs and budget are minimal, don t consider a used computer that does not run at least Windows 3.1 or Mac System 7. If the computer is too old, you will waste a lot of time learning outdated and hard to use software. There are some useful web sites that compare prices and explain features. You should be able to get a more honest appraisal of the system and some help with setup and use. Ask questions and make a list of your likely needs before you buy.
Software is what makes a computer useful.
If you have specialized needs, confirm that the software will run adequately on the computer you pick out. Often a computer comes bundled with software. This can be valuable if the software fits your needs. Educators can get deep discounts on some software. You can usually install it yourself in a couple of minutes.