Hey, I use JBL Control1 with external amp, but I think that you can have way better sound quality with those JBL LSR 305, than M-audio, or KRK, med and low are IMO better !
I would suggest that you try the secondary pc encode with your laptop (or a pc that you can borrow), to see if your primary computer workload is less impacted.
Hey, defintely the i3 setup, the gtx 1070 is great for the price (maybe overill), but overall perf look way better. And the CPU can be easily upgraded for next gen games !