If looking for a compact Mac in 2020, unless it has been torn down, recapped and/or rebuilt there is very high chance it will not work. That's why the ready to ship, rebuilt machines cost substantially more. If you are looking for a "fixer upper", you can probably get something cheap but be prepared to buy another for parts - it's not unheard of for people here to merge together two Macs into one. As Might Jabba mentioned, the SE/30, while a nice Mac, is probably one of the biggest headaches to maintain and repair - the SE, Plus are usually much more solid. Classics less so if unmaintained (nearly all are decimated from old batteries unless checked).
When picking up, even if you can't get into a Mac inspect the vents and underside for rust/corrosion/battery leakage - it's sometimes subtle. I've walked away from a few sales from generally unwitting sellers who are selling an "untested", visible pristine Mac but it's clear the innards are toast once you peer around vents and notice orange/brown discolouration.
JB