Mesh, your USB/SPdif converter strips timing from the data. In asynchronous USB data is delivered in packets at its own timing that has no connection with clock used by D/A converter. Furthermore, Pure Power prevents jitter induced by noisy computer supply. Pure Music is fine, since you already have it. 8GB is not necessary now (I run El Capitan on notebook with 4GB), but is probably a wise choice for the future. Yes, you can use the same USB, since at this point only data (and not the music) is delivered to your converter (slight delays are not important). Situation is different on SPdif side where timing is crucial. I believe that your converter has both Toslink and coaxial outputs. Normally coaxial should be better but try both if you can. It is system thing and in some cases Toslink works better. If you use coax, try to buy or build one, that is very short, less than 12". The shorter the better. If you cannot do it then get 1.5-2m. In first instance you won't have reflections in the cable (from the end of the cable) affecting transition while longer >1.5m cable will delay reflections making them miss original transition. Try different digital cables. It is characteristic impedance matching and more expensive cable is not necessarily better.
Get the least expensive slowest new MAC. SSD is not required but it is quiet. Use power filter for your DAC if you cannot use different outlet (different phase) for computer.
