I've thought about getting an older cheesegrater mac pro just to case mod it for a modern hackintosh... they're still going for a bit more than I'd care to spend just to rip the innards out and still have to put more parts to modify modern atx hardware into it.
I keep thinking about it all the same... my 4790k hackintosh is aging well, and when I upgrade later this year may find a new home for it in one of those cases.
I just got rid of a 2006 Mac Pro since I'm moving in a few months, but it was pretty great. There's a ton of aftermarket upgrades you can do and even mine, the oldest Intel model, could run El Cap. The subreddit /r/macpro has a ton of info and if you get a 4,1 or 5,1 it'll go up to 12 cores and run Mojave easily.
I keep thinking about it all the same... my 4790k hackintosh is aging well, and when I upgrade later this year may find a new home for it in one of those cases.