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Work backwards from the workload. A machine that stores files, runs a handful of containers and streams to two televisions needs very little processor and quite a lot of disk. A machine hosting several virtual machines, a database and a game server needs the opposite balance, plus memory to hand out. Writing down the actual list of services first turns an open-ended parts question into a short one.
Then account for time. This is a computer that stays powered for every hour of the year, so the components that matter are the ones that behave well under that condition: a power supply efficient at low load rather than rated for a peak it will never reach, fans that stay quiet at low speed, and disks specified for continuous duty. Peak performance is a far smaller part of the decision than it is in a gaming build.