I'm assuming your concern is hardware longevity not obsolescence. Obsolescence will happen before 20 years because of speed, RAM and disk capacity inflation, an

I'm assuming your concern is hardware longevity not obsolescence. Obsolescence will happen before 20 years because of speed, RAM and disk capacity inflation, and compatibility with latest operating systems. Servers that have survived 10 to 15 years are quite common, 20 a lot less so. Probably the easiest way would be to buy 2 "fairly good" identical servers, run one of them, and keep the other one off for spare parts. By fairly good I mean a branded server (not entry point) with dual power supplies and redundant disks. Alternatively, you can wait 7 years and buy spare parts then. Unless things change dramatically, it will be easy then to find parts for a 7 year old server, and, if used, cheap.