I do basic video editing, transcoding, front end coding and graphics, and usually push my machine pretty hard. But not for really long stretches. It’s bursty work, so an Air is ideal. If I were to need a desktop replacement I would get a 15" Air with 24 GB RAM.
Long compiles? Renders of 4k video over 30mins and on the clock? Get a pro.
This is an extremely regional problem to solve. Where I am, which is a village and exurban-ruural, you would go to the electronics recycling depot and see if they have any choice items. Also you could call the various independent pc repair people to see if they have anything no longer supported but functional for free or cheap.
Also there’s various thrift stores that sometimes have computers cheap.
The closest big city is Vancouver so to curb cruise there I would pick upper middle class neighbourhoods with alleyways, and drive around on garbage collection days. I wouldn’t really dumpster dive unless I knew of a likely source from hearsay.