I'm building a PC. Have questions

Hello everybody. My current PC is very old, with an i7-3770. It has served me well over the years, but I am long overdue to build a new one. I have a large collection of physical games that I have been collecting since Ps1, going all the way up to Ps4. All of my consoles still work so I don't really need to emulate anything...but I want to. And that's why I'm here. I want to build a relatively future proof PC that I can play my games on and preserve my collection. Apparently the Ps3 is the hardest system to emulate, and apparently the best CPU for the job has been handicapped by the manufacturer. Sort of. And thanks to that, apparently most motherboards that will run Alder Lake processors have been handicapped at the BIOS level. ... So I have questions.

I already know that the newer i9-12900K chips have been handicapped on the chips themselves, and I know what to look for in order to find a good one on eBay. What I need to know are the PCs or motherboards that have not disabled the ability to disable E Cores and/or enable AVX-512.

First off, are there any stock used PCs that I could buy for relatively cheap on eBay? I was planning to buy an Optiplex 7000 before I learned about all of this. Is there a chance that I could find an original with a stock BIOS that still allows disabling E cores and enabling AVX-512? If there is a chance of finding one I would ask every seller on eBay to test that for me until I found one.
Are there any similar mini tower or small form factor PCs that will or originally did allow AVX-512?

If I have to build a PC from scratch, What is the best gaming motherboard and case combo? I don't want a massive gaming case with unnecessary lights and a 50 fans. I would prefer a mini tower or small form factor. I plan to use at least one internal 3.5" hard drive, and an optical drive. So whatever case I buy will need to have the option to install those.

What do you guys think about Xeon processors?

If you want an Alder Lake CPU with AVX-512, please see this reddit post: https://www.reddit.com/r/rpcs3/comments/...nd_how_to/
It is quite unlikely that any prebuilt PC would be able to enable it unless it was a custom built PC you are buying from someone else.
I personally have an MSI Pro Z690-A D4 which can enable AVX-512 on my 12900k on an older BIOS version, though as for the case you would be hard pressed to find one with everything you require as most manufacturers do not make cases with 5 1/4 inch bays anymore, much less smaller form factor ones. Since most people have no use for an optical drive these days I suspect it is highly likely that any small form factor case will not have one as a space saving measure. There are enclosures you can buy to run internal optical drives externally so I would suggest you look into that before wasting time trying to find a case that does everything you need.
Xeons are not good for RPCS3, they may have high core counts but they generally also have much lower clock speeds due to the high core counts, and RPCS3 can only use so many cores and threads for actual emulation. Mullti-socket solutions are out of the question as well, as the inter-CPU latency is too high to offset any performance gains that may come from it, if any at all.
Alternatively, if you don't want to mess around with older BIOSes and specific hardware revisions, AMD's Zen 4 chips are also good for RPCS3, they all support AVX-512 by default and they are still very fast for PS3 emulation. I am unsure if there are any Micro ATX or Mini ITX motherboards for Intel that would be able to unlock AVX-512 but I know that the AMD ones can all use it just fine.

If you want an Alder Lake CPU with AVX-512, please see this reddit post: https://www.reddit.com/r/rpcs3/comments/...nd_how_to/
It is quite unlikely that any prebuilt PC would be able to enable it unless it was a custom built PC you are buying from someone else.
I personally have an MSI Pro Z690-A D4 which can enable AVX-512 on my 12900k on an older BIOS version, though as for the case you would be hard pressed to find one with everything you require as most manufacturers do not make cases with 5 1/4 inch bays anymore, much less smaller form factor ones. Since most people have no use for an optical drive these days I suspect it is highly likely that any small form factor case will not have one as a space saving measure. There are enclosures you can buy to run internal optical drives externally so I would suggest you look into that before wasting time trying to find a case that does everything you need.
Xeons are not good for RPCS3, they may have high core counts but they generally also have much lower clock speeds due to the high core counts, and RPCS3 can only use so many cores and threads for actual emulation. Mullti-socket solutions are out of the question as well, as the inter-CPU latency is too high to offset any performance gains that may come from it, if any at all.
Alternatively, if you don't want to mess around with older BIOSes and specific hardware revisions, AMD's Zen 4 chips are also good for RPCS3, they all support AVX-512 by default and they are still very fast for PS3 emulation. I am unsure if there are any Micro ATX or Mini ITX motherboards for Intel that would be able to unlock AVX-512 but I know that the AMD ones can all use it just fine.

Thank you for responding and for the useful info.

As far as I can tell, motherboard sizes seem to be standardized. Does that include mounting screw hole locations on the boards? If so, do you think I could mount the board in an old case? I have an opportunity to buy an old Optiplex 990 mini tower for cheap. That would be a perfect 'sleeper' case and it would give me all of the functionality I want except for USB 3.0 on the front panel, and there might be a workaround for that.

I have come to the realization that the i9-12900 uses too much power and produces too much heat to be used in a SFF case. AMD CPUs are out of my price range. I could wait for the 15th gen of Intel CPUs which will have AVX10, use less power and run cooler, but they would also be too expensive. Minisforum motherboards look perfect because you get the Motherboard, CPU and CPU cooler all in one package. That would make it affordable, but they seem to be a bit underpowered.
https://cpu.userbenchmark.com/Compare/AM...vsm2064608
The i9-12900H-something doesn't seem to be quite as underpowered, but I have no way of knowing if AVX-512 can be enabled.

With everything that a Minisforum motherboard comes with, I think I could wait for the next gen of CPUs. Minisforum motherboards don't come with SATA ports but I found M.2 to SATA adapters. If I could bolt it into an old Dell SFF case, that would be perfect. What are your thoughts?