I think the animosity in this thread is slightly overkill, the guy says he's a noob and because there's a search function doesn't mean it's easy as pie to find the right info nor understand it all.
BadMouth gave the best lead here, basically look for the recent generation CPUs delivering as much GHz as you can afford, preferably one w/ at least 4 cores.
(the Single Thread Performance he mentions is a popular rating delivered by passmark software at cpubenchmark.net, you can check their website database, but afaik they don't quote top boost or overclock performance, so the usefulness is limited)
If money is not a problem get one that reaches well above 4GHz on boost (4.5GHz or more) check that on ark.intel.com
If you look to save a bit get an unlocked model (ending in 'K' or the upcoming 'KF') so you can unlock the multiplier and run all cores at max performance relatively safely (requires a mobo fit for overclocking, a beefy PSU, and good cooling fan)
Don't bother wich 'i' series it is, because a i7 is stronger on paper doesn't necessarily mean it gets you a significantly better performance VS. an i5 or even i3 that overclocks well.
The i3-8350K for instance is a strong performer, some users even manage to push it to 5GHz, although you might not reach exactly this far with your own rig, it's still a clue this processor overclocks well and you could get a result rather close to that. You'll find some higher-end i7 or i9 than can't even overclock by +10%, which means they're less value for the money.
You'll worry about the GPU and RAM after you get this: CPU+mobo (and cooling fan if you go the overclocking route) right.
Yet as you were properly advised overall in the thread, even with the best rig you won't be able to play everything, nor everything right/perfectly. Emulation is not just a matter of power, a high-end PC will only allow you to play a bunch more of the heavier games and emulators.
I like to compare the hobby to audiophile madness, where a couple hundred dollars rig will contribute to 80% of the overall quality, then trying to reach in the 20% remaining will cost you several hundreds more up to thousands while in practice you'll only get 5%/10% better sound.
In any case take your time to think the basis of your PC well (CPU+mobo and fan accordingly)
You'll always find people to tell you a cheap/old/used PC is enough or even a Pi, but they're often those who play mostly old school titles that don't require a high performance rig at all. Likely you'll find those who will tell you to purchase a NASA-worthy supercomputer.
That's why some legitimately ask "what do you want to play", though it's impossible to tell exactly which is the perfect sweet spot configuration, but I'll add my own 2c to conclude:
- indeed if you don't want to regret buying a too-low-end PC that will disappoint on demanding games and emulators, don't go full crazy spending a year's worth of salary, but don't cheap out too much either, and think about the following carefully: for how many years do you want this computer to be relevant? emulation evolves, and not always in the direction better optimization/speed, overall it's something that requires more and more power over time, MAME in particular is very much like that.
In any case thinking about the proportion of games a decent computer will allow you to play well, you know the money you spend on it represents only a minuscule fraction of what the real consoles+games and arcade boards cost (a single arcade pcb exchanges for several hundreds or thousands).
PS: unless you already have a display and don't plan to buy a new one for your rig, you might want to think your picks and budget including that too, for instance if you are interested in FreeSync or G-Sync then you cannot buy just any monitor nor any GPU, they'll need to be compatible.