It’s actually my main issue : finding the right point where the cost of a used computer is no longer competitive compared to buying new or building.

On a personal level, even though we’ll have to deal with currency exchange rate and cost of life etc., where would you put your limit?

While the cheapest office desktop bottoms out at about ¥12,000, the first server/workstation computers seem to start at about ¥40,000 (a Precision 5820 with a Xeon W-2102, 16GB of RAM, a 512GB SSD + 500GB HDD https://www.pcwrap.com/item/detail/1324009 )

Where to draw the line, I’m not sure…

Indeed, and even the case can be tricky. I think you sometimes have to drill the case if you want to install a standard motherboard.