Which Mac would you get to run Xcode CI server?

I wanna get a mac to run Xcode CI server. It will mostly be running headless in my home network. It will run tests for both Mac and iOS projects.

What should I get? 2nd hand 2012 Mac mini (just so I can upgrade ram and hd)? 2014 Mac mini? Just run Xcode server in my rMBP?

What's your solution?

Thanks
Bill