I think it depends a lot on what your end product is going to be.
Desktops aren't really something people buy a lot of these days what with tablets and laptops out there that fill 90% of people's needs. Take an old desktop, replace its hard drive, install an OS, and you still have to convince someone to buy it from you for $300 or so, that they can spend on a new cheap (or even OEM refurbished) computer or laptop.
Fixing up broken laptops has more appeal, but is more labor intensive. Were I to do it, I would target corporate auctions and buy a pallette of one model and start there.
To do it properly you would need a re-sellers Windows license or distribut a free OS.