You keep mention issues with the D300, and the D500 being more reliable, but not sure where you're getting that? Any GPU is prone to failure - I haven't seen anything to suggest the D300 is any less reliable than any other GPU? (and no, generally speaking this forum is not a "source" - it's a joke as far as any kind of objective discussion on the nMP)
I have the D300 (purchased early 2014) and have been sporting two 4K displays for a couple years now, and it's a great setup. A friend has the exact same setup and also no problems.
The D300 is not great if you're a gamer, 3D modeling, or anything that's GPU intensive. But I dabble in FCPX, and it works absolutely fine for light video editing.
Since you've repeatedly indicated that you do audio, the D300 is perfect for that. The nMP makes for a fantastic audio workstation IMO. The CPU will just chug along, super quiet, lots of connectivity.
Good luck!