I moved one of my computers at home from my downstairs office to my upstairs office. The prices for used or rebuilt computers varies. Once you see a system that you are interested in, compare the price with that of a new computer. Finally, as the old saying goes, you get what you pay for . Don't expect the old 386 or 486 class computer to run as fast as the Pentium class computer that you have in the office, it won't and never will no matter what you do.
Older computers are usually upgradeable to a point, but there is a limit.