486 DX4 100 will play MP3 😉

If I went with an early ATX-type power supply, what brands are recommended?

so I just built a 486 over the weekend and have 2 pointers (if you're ok with spending the cash or have great deal finding technique) - definitely do not compromise on a VLB video card, be sure to check benchmarks. I got a Trident ISA card out of nostalgia and it does not look good on an LCD at all as well as being so slow (about 12fps in DOOM). After a day of using the Trident I have a cirrus logic VLB card on order 🤣.

Another thing would be that I didn't get a motherboard with a Ps/2 header. I should have forked out the little bit extra to get ps/2 mouse support as ball mice (and even the optical serial mouse i have) are still a pain in the ass - though once serdashop is open to send outside of the EU I am buying their serial-to-ps/2 active converter, so there is -an- option. If using a ball mouse is going to be fun for you or part of the nostalgia then don't mind me 😀

As for ATX power supply I can't help you there, mine's in an AT case with AT power. good luck on your build!

Finding 486 with PS/2 ports can be difficult, not that many motherboards of the era supported it (and those that have both VLB and PS/2 ports are even rarer, FIC-486-G**, and DFI G48-V** , both based on VIA chipset are the ones that I am aware of).
Brand name systems usually have PS/2 ports and integrated video card on board, and if you can find one in a good shape and decent price go for it (though they most often go with ISA only or ISA/PCI slots, VLB is again a rarity , IBM ValuePoint 6384/D and IBM ValuePoint 6387/T however do have it).

PS/2 ports are a very convenient thing, but this really is about with mouse - keyboard uses the same protocol, and simple DIN > PS/2 adapter will do the trick, they are quite common, and you can do one yourself eventually. Mouse uses different protocol, and while RS > PS/2 adapters do exist, they work only with dual mode mice . It all depends on how much you care about having a modern wireless, optical mouse in this retro system 😀

As for I/O card you may not need it at all, it you find motherboard wit integrated I/O section. If you are however going to use a separate I/O card, I would go for an ISA rather than VLB card – from my experience they are more reliable and work better with modern HDD replacements (CF and SD cards). Yes, they are a bit slower, but in DOS this doesn’t matter much.

32 MB is more than 486 needs , but if you have more, it never hurts 😀