I’ll let someone else give you specific specs, but I want to give you two pieces of advice:
- You can get by with an older piece of Mac hardware,
- BUT -
- Don’t get something too old because, iOS is frequently updated. This drives frequent updates of xCode. That in turn requires newer and newer versions of Mac OS. Mac OS eventually drops older versions of hardware… finally leading to your hardware being too old.
Finally, I’m thinking a used 2014 mac mini would do the job. This is actually the most recent version, but I think a new one is coming out this year (2017). A 2014 should last you for a few years before being too old for the latest Mac OS.
PS You might even be able to get by with a 2012 Mac Mini, but don’t be surprised if those are actually more expensive than current ones. AFAIK they are in some respects better than the newer ones.
Interesting and useful links:
http://www.macrumors.com/2014/10/31/mac-mini-buy-which/
https://en.wikipedia.org/wiki/Mac_Mini