So, to start I will stress that this is mostly just my opinion and that I only have personal experience to go on. Also, I have very little knowledge of Unreal yet, but I thought I would throw in what I do know.
First, on hardware. I have used a fair amount of hardware, but I have found that for longevity Intel processors paired with NVIDIA GPUs works great. Now, obviously these are more expensive than their AMD counterparts, but like I said they seem to last and stay relevant for longer. If you do not plan to upgrade every year or two, then I would recommend Intel/NVIDIA. Otherwise, if you are fine with the performance specs of AMD and do not plan to run the hardware long-term, then that will probably work for you.
As a reference, I am running Unreal on a laptop with an Intel Core 7 and Geforce 1060 that I got three years ago, and when I hit “auto” on the scalability settings it puts everything at “epic”. So, that is what I mean by longevity and relevance.
As for GPU usage, I do not know a lot of the specifics, but I did read in the Unreal documentation that light compilation seems to be bound to the CPU. From what I have read from third-party sources most other processing is also done CPU-side. It seems only the dynamic rendering is done GPU-side. That said, a good GPU would allow you to stress your game graphically and see where rendering bottlenecks may be.
In general, also consider what your players may be using when developing games. If you go all-out on a development rig and only use it for testing, then you may not get a representative results of performance for the end players. For me, I have my laptop for development and an older desktop that equates to a mid-range computer by today’s standard to test PC builds on.
Hopefully this is helpful.