I'd plan out what machines do what according to their drive sizes rather than finding out the hard way that one of them only has a few GB spare that I used as a mail server. Certainly document what I have going, if my machine Francesco explodes one day it'll take months to remember what was actually running on it.

I'd also not risk years of data on a single SSD drive that just stopped functioning for my "NAS" (its not really a true NAS just a shitty drive with a terabyte) and have a better backup plan