Calculate what individual type of specs a pc needs installed work each individual part

I am currently rebuilding second hand PC's. You know, the old 2006 dells you find in recycling centers and scrap yards. So far it is proving very successful. If I want to install a new CPU or a New hard drive I can do that with no hassle.

Almost no hassle. But how do I know the minimum amount of ram this new CPU depends on? Or what CPU or hard drive this new graphics card depends on? What if I need some certain amount of hard drive space for my pc to run correctly?

I want to distribute these PCs and need a way of calculating what each part would depend on, essentially calculating the amount of ram per cpu ghz or something. All help is appreciated.

Thanks