If you are doing stereo, quad-core with high as possible clock speed is good. If you want to do lot of stuff with Roon while playing music, six-core could be a good option too.
Only to limited extent, but something like 8 GB is good minimum baseline since Roon will likely eat quite a bit too. But HQPlayer likes fast and wide RAM. So keeping CPU’s memory channels populated with fast RAM helps.
Not any, it should be fairly fast. If it is significantly slower than the main CPU, offload will likely cause more harm than good. The new 1060/1070/1080 should be good depending on what you plan to do.
It is really hard to give any generic response, because there are so many different possibilities with each having different requirements and optimal configuration.