Most socket 5 boards are pretty basic and are nothing exciting but just as good for late dos as socket 7 until you need something with more grunt. Socket 7 is just more convenient and flexible. Should be able to find some ATX boards on eBay if you take a couple of hours to scrounge through all the listings below $60.

I have Asus TX97-XE (soc 7 ATX) for sale if you want. Considered one of the best soc 7 (not SS7) boards, have many CPU's to go with it too (166-233 MMX, 100-133 Classic etc).

I would go for a Socket 7. You have more choices there. Socket 7 mobos also survived long enough to see the ATX standard being introduced, so there are some Socket 7 mobos that can use more modern PSU and cases. It'll be easy to find stuff and build around.

I personally went for a Super Socket 7 just even more options - I can go as low as a Pentium 100 all the way up to a K6-3+ 500 Mhz. Also because it's ATX, I could use a modern PSU and case. I believe it's an Asus P5A ATX. Currently running a P-233 at 250 MHz (100x 2.5).

I have a huge soft spot for HX chipset motherboards, I kept mine from back in the day when they were new (M-tech to be precise AT format).

If you go the socket 5 approach I would just get a premium brand that uses socket 5.

Standard ATX late era socket 7 boards are just boring to me.

I really don't care about atx and I can easily add -5v and rewire an atx psu.

As for hard to find parts, I was thinking of going full eisa and don't care about about rear parts. How does stability and support play in.