How long do you want it to be "serviceable"?

The supply of specific brands/models of old hardware eventually dries up and gets hard to find.

When willing to replace with similar or even completely different stuff for a desktop build, it becomes much easier.

Your geographical location also plays a role to some extent. Stuff easily available in the US may be almost impossible to find somewhere else.. and vis-versa.

Slot 1 machines with i440BX chipset are often easy to work with (ATX form factor so it will fit pretty much anywhere), still easy to find, they have a wonderful compatibility with a lot of things, you don't have to set jumpers to a certain position everywhere when you change the cpu, even if they miss their I/O shield, most of them use the exact same one

You might encounter few issues with things like memory sticks only showing half the capacity, hdds limited to 32GB maximum (can be fixed with BIOS update on many boards), but they still have a range good enough so it shouldn't bother you too much. You can install CPUs from 233MHz up to 1GHz offcially (and 1.4 with mods/slotckets), you can slow down early pentium 2s to 133MHz, disable caches, install a via cpu, play good old windows 9x games which IMO are by far the most difficult games to run on modern systems.

If you aren't worried about great DOS compatibility then Socket 370 intel 810 or 815 chipset machines are often very good too and should be easy to find. GeForce 2 MX and 4 MX cards should be easy to find, as are tons of GeForce FX 5200 cards. Sound cards are a mixed bag, but for Windows non-oem Creative cards are usually fine and easy enough to find.

Edit: I also seem to find lots of Radeon 9250 cards, and they work well too.

I'm actually not too worried about dos support, because I didn't really grow up with dos and I know dosbox is pretty adequate. PCem/86box is sorta getting there. "Similar" hardware fits the bill, I wasn't looking for exact common hardware, just common hardware. So basically if that's AT/ATX boards and Pentium IIIs then that's what I'd shoot for.

That being said LHN91's response is very helpful. I was thinking somewhere around geforce 4s, AT cases, and pentium III boards. Just wasn't sure what chipset exactly.

[EDIT] Yup, there are about 100 ebay results just for Socket 370 ATX Boards with AGP.

Glad I was helpful 😀 I maintain a set of 8 Windows ME machines for my in-laws. They want to play mostly 9x compatible shooters and RTS games, Outlaws top of the list. They're a mix of Pentium 3, 4, and Socket 754 Semprons with mostly GeForce 5200's and Radeon 9250's.

Reliable and easy to find at the recyclers is pretty much goal number 1 😀