I'm up for adopting anything smaller than a minicomputer I guess. Though in era, I doubt I'd go earlier than mid 70s. 8080, 6800 class up really. But I won't pay much for anything. Entire 8 bit collection at the moment is just a CoCo2, and 6800 and Z80 CPUs, and maybe some S100 boards if they turn up again. I am not finding the CoCo2 that interesting, maybe need to expand it and put OS9 on it for kicks. Got some Amiga stuff A1200 and 2xA500, but they don't get out much, seems like I'm always looking for that 8x4ft desktop to have the ultimate Amiga setup, set up. (They kinda sprawl, you can't stick the system unit under the desk and use a compact keyboard, well unless you got a "big box" paradoxically.) Then PC I've got from 8088 to i7, gaps include 386DX and PPro. I guess I have optimum fondness for 486 class, super7 and socket A. I like PC/XT class quite a bit, pure pure DOS machines. 286/386 slow 486 kinda feels like an era where things sucked at GUI. Maybe it's my Amiga streak.
Started home computing in 1982, so guess it's mostly what I grew up/older with. With maybe a feeling like I want to fill a gap with a "serious" CP/M machine from ~1980-1985 timeframe. But about the only thing I'd spend "serious" money on would be a big box Amiga, but I gotta get a deal still. I don't even like paying more than $50 for a modern part, (Single used part, think that's as high as I have been recently, for an X6 1090T) and for retro, generally do my accumulating before stuff gets to stupid eBay prices, though there's lucky finds still, like a banshee for under $15, a pile of slocket stuff for $30 etc. I don't know if I'm really actively looking for anything, I stay out of pay anything frenzy mode and browse frequently to see what turns up. Don't think there will be a top end cutoff per se, though I am currently dropping my personal divider of retro vs modernish usage down at dual core, but that might go to quads plus soon.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.