I'm sure you'll get a wave of good responses here, but I can speak rather highly of the old (beige) Dell Optiplex models. I have one running a Pentium 200 that is pretty solid (and cost me 25 or thereabouts) that easily fits under the monitor of my primary machine. Granted, I'm mostly doing DOS gaming on it, so I don't need (though I would like) a lot of expandability. Not being sure what you're after but seeing you're ok with a laptop, I'd assume it'd do the job.

Also, this line went, I believe, up to Pentium III and since they have integrated graphics and sound, they're pretty easy to get up and running and upgrade later. So eBay might yield some gold for you there...

Ohh damn! I had no idea they went any smaller than microatx in that era. I guess I need to be looking at more proprietary builds. The pentium III optiplexes look perfect, they even have AGP/PCI support. Looking at a gx150 or gx200. It seems the only benefit of the gx 200 would be the nostalgia of beige, 🤣. Thanks so much though.

{update} It seems the only option is a gx150 sff for $100. All the others are broken, untested, or mid towers. Any other recommendations before I take the dive? Part of me still would like a laptop like an inspiron 8100, but I feel that old laptop hardware would be difficult to maintain, what with taking about 40 minutes to get to the cpu depending on brand and lack of manuals.

One interresting thing you could do if you wish to go more retro: a complete PC in your 5.25" bay. It can be anything from 386 to Pentium if you manage to find a good board for cheap.

There's an article on the subject. The text is in Russian, but there's a lot of pictures and Google translate can help you with the rest.
http://www.phantom.sannata.ru/konkurs/2011/kt1111.shtml