Are those specs fine for a good emulation?

Prebuilt PCs are garbage, look for a shop that builds custom PC part lists and have them build a decent one for you.

I'm talking about 30fps stable for most games, which is the target of most PS3 games.
If you wanted 60fps on the demanding games you'd have to go for the highest end chips like 8700K or the Zen2 Octa Cores.

Online or physical shop?