The Benchmark isn't the most musical DAC, you can do better for sonic enjoyment. The latest one has DSD, which is way beyond what you'd need for AIFF files. Recommend shopping for sub $500 DAC's (that's what I did for ALAC files).
I'm a computer dunce, so have only looked into software. Again for AIFF I'd look at something like Decibel (a $35 iTunes add-on that is supposed to be simple to install/use).
I'd say, what do you have to lose trying to use your old MacBook? I'm sure it'll run iTunes. Even if you decide later to go to a Mini, you'd want a external HD for backup anyway, so you'd be out nothing for trying. Don't know how the MacBook sound quality would compare to a Mini, but running off battery must be an advantage.
I run a 2014 non-dedicated MacBook Air via a 3 meter USB cable from my listening chair to feed my audio system. I have a small library that fits in it's SSD. I use the MacBook Air everywhere including at the listening chair and only charge the battery when it drops below 20%. So using my MacBook Air requires plugging the USB in (and sometimes the power cord). From my single listening chair it's fairly easy to accommodate the cable(s). Not as slick as a iPod controlling a headless Mini (of course display/keyboard/mouse is still needed for setup and troubleshooting). You'd have to decide if you're a tinkerer or can get by for extended periods without messing with the Mini. Personally I just didn't want another computer.
