Almost all calculations perform on GPU. So, you should take the best one. Even NVIDIA's flagships, like 2080, 2080 Super can't convert 720p natively, only 2080Ti.
CPU is used only at the first and the last stage: converting to/from images, so, in this case, CPU is not so critical (I suppose, you are not going to buy 2080 Ti and AMD K6 :-) ).
Viewing post in Great Tool but which hardware do I need?
I had to look up what an AMD K6 is...
I'd not long started school when that came out!
A bit weird how I know the Intel Pentium chips of the same age though. Maybe because my school PC's probably still had the original Pentiums by the time I started using them! We've been taught the frustrations of outdated hardware from an early age I guess :D
PS. good joke ;) I wonder where you'd plug in the PCIe?