Every so often this question pops up and it's never a super simple straight forward answer of "buy X product".
I'll just repeat what I always say, buy the best modern CPU you can afford / willing to spend on. It doesn't matter right now if you buy Intel or AMD but if you are going older generation like pre Ryzen series AMD then you want Intel. You can never have too much CPU power with emulation, you can always push the extra CPU power into settings (frame delay) in Retroarch and Mame to reduce input latency.
Don't stress too much over the graphics card, they aren't a major factor in emulation unless you start talking about the newer systems emulators like RPCS3 and Cemu for example. Even a newer AMD Ryzen APU would be enough.
Hard drives are cheap, get as much as you think will need for your collection. If you are going for full sets (especially CD and DVD based systems) then you will want A LOT of drive space. But if you are only going to have hand picked sets then you can get away with much less space. Don't bother with having your rom collection on an SSD drive, an old fashioned mechanical drive is perfectly fine. Though I would highly suggest an SSD for the OS and maybe even Launchbox but keep in mind if you have a large collection and get all the media available then you may run into space issues on an SSD unless you are willing to splurge on a larger one like 2TB - 4TB.