I would go for minimum 500gb storage, 8gb ram, i5 or better processor. Unless you want to move into more professional recording dont listen to the garage band naysayers. If you are just jamming with mics hooked up and having fun, garage band is fine. I did some brief sleuthing and i found probably the mac mini i would get in your shoes, but obviously i dont know your budget.
https://www.amazon.com/Apple-Mini-MGEN2LL-Desktop-Refurbished/dp/B017A74RW2/ (Not affiliated with seller or anything promise, but i think i would go for the 2014 model since it will be supported at least two more years and will be the lowest price for something that will work its ass off for you)