I think you are on the right track as to finding a donor bike. I rarely if ever buy new parts for older MTBs. I find donors frequently. Better cranksets will have chainrings that are bolted on, not the cheap riveted on rings. Also hit garage sales, I see quite a few MTBs at garage sales. I finished my wife's MTB with a $5 donor from Goodwill: crankset, rear derailleur, wheels, freewheel, etc., all came from that $5 donor.
Happy hunting.