Going to upgrade to Cubase 10 Pro with a new PC build (on an i5-2600k now). Mainly do VSTis, not much direct audio recording. Would an i5-8400 be good enough or would there be a significant difference from i17-8700k? Or any other suggestions? Thanks!
If you can afford the i17-8700k, go for it, it is slightly more future proof, just since it is slightly faster.
Always use an i7 build for Cubase over an i5 if you can afford it as Cubase will use all the cores of an i7 which means more VSTis at once for you.