If you monitor through your audio interface's built-in near-zero latency digital mixer (which most interfaces have these days, and many have built-in reverb for "wet" monitoring while tracking) you won't have to worry about throughput latency. Regardless, if you operate your DAW with a small buffer (64/128) you will be fine regardless of which Mac you select.
I am a pro doing heavy audio recording (tracking and mixing bands plus lots of sound for picture and production work involving multiple VI's) on a hex 3.33 4,1>5,1 and have yet to max out this machine, even with heavy VI use at buffer of 128. If you are doing straight audio work, there is no way you will able to overload a nMP hex, I don't care how many compressors, EQs, Altiverbs and FX you use. Only if you get into full VI orchestrations is there a chance you will need more Mac. But you will first need more storage (SSDs).
In fact, based on your plans, I'd be willing to bet you would never even push the nMP base quad. Put the extra money toward a nice mic and mic pre and some great monitor speakers.