At the start of classic I was using small form factor HP/compaq business machines I bought off ebay. All of them were i5s with one 2500 sandy bridge and the rest 3470 and 3470s models with 4gb ram and integrated graphics. A couple of them had HDDs that were dying but it was good enough for one client (anything above 3 clients was painful). Giving them 8GB ram allowed for 5 clients to run "decently" with optimization. When I slapped some HD7570 1GB gddr5 cards that I bought for $9 each (ebay) it made 5 clients without optimization possible.
I upgraded one of the i5-3470 systems to an i7-3770 with 16gb ram a gt1030 2gb gddr5 card and a $21 new SP 512GB SSD (on amazon). That machine runs 10 clients without optimization pretty well although the orc event does get laggy. I wouldn't recommend a gt1030 these days unless you can get one for super cheap and make sure it's not a ddr3 model as they are significantly slower. I only bought the gt1030 because it was 2019 and SFF had mostly expensive options. A rx6400 is VASTLY better and not much more expensive than a gt1030 new. I recently assembled an i7 3770 SFF system for my sister's young kids and the rx6400 in it gives almost 2x the FPS of the gt1030.
I still have one of the i5-3470 machines running with 16gb ram and another Silicon Power 512gb ssd with a hd7570 still. It runs 9 clients pretty well but it's clear the GPU is limiting it and the CPU aint happy with that much load (4 cores but only 4 threads). So optimization is the name of the game for some of the clients. The 8 threads of an i7 would help but a better GPU would help more.
I recently assembled a machine to replace my old fx6300 system (mobo issue). Bought a dell optiplex 7010 motherboard (773VG) off ebay for $16.32 shipped and a xeon e3-1230 SR00H for $12.71 shipped. Tossed that into the case of the fx6300 system and hooked everything up. It has a generic HP 8gb ddr3 (1600) stick and two ripjaws 4gb (1333) in it and it's running the ripjaws in dual channel fine while still using that single 8gb stick. It's kind of amazing but OEMs do mix memory stick sizes and such (6gb setups etc). Ran it 24 hours with prime 95 and it's been running a week straight with 10 clients no errors. It does get mad that there's no front panel connector, no case temp sensor or back case fan hooked up but F1 gets past that at boot up (I have plans to fix this for cheap). In benchmarks the i7-3770 CPU is about 3% faster simply because the i7 clocks about 300mhz higher. I'm planning to buy another xeon for my i5-3470 system since they are WAY cheaper than an i7-3770.
So while I might not of directly answered your question I think I at least provided something of value with my examples. Some could be cheaper alternatives unless you need the steamdeck or ROG ally for something else.