NAS? not so sure
I'm not so sure about how good old PCs are for a NAS solution. Firstly do you really want to trust your data to drives and power supplies that are on their last legs? Secondly older PCs tend to be power hungry and they are powering a lot of additional stuff you don't really need in a NAS, plus NAS drives tend to be on 24/7 so power costs quickly mount up. If you don't care about either of these then fine, but you can get very functional, low power 'home' NAS boxes for a lot less than the price of a desktop PC.