New
#1
Before you buy a used computer...
Same as buying a new computer. Write down the model and specs, and then go home where the salesclerk can't pressure you. Look it up on the web.
One poster had bought a refurbished Windows 7 desktop and was happy with it. Upgraded to 10 and had all kinds of problems. Turns out that it is an XP computer that had been upgraded to 7.
Any rig that can run Vista can probably run 7 or 8, and then be upgraded to 10. And since some XP rigs can be upgraded to Vista (Usually a bad idea, by the way), they can be upgraded to 10. The different architecture of the internal hardware will probably accept 10, but it isn't designed for it.