okay so suppose I want to buy a cheap (used) android that can be updated to run arbitrary current apps on my home internet without a cell plan, what do I get

I don't want to mess around with my regular phone, it needs to stay functional in case someone wants to hire me for $500,000 a year (it could happen) 

 Bf got an old pixel because you can wipe it and install a different OS. He's very pleased with it. 
Make sure you check the available OS and their model compatibility. 

 Second pixel. 6a or above