Your best bet might be to use a laptop as the basis. They are already designed with power efficiency in mind, and you won’t need an external screen and keyboard for local problem solving.

I would also consider having a raspberry pi 3 or similar as a companion. Services that must be up all the time run on the pi (e.g. network admin). The main computer only gets kicked out of sleep mode when required. The pi 3 needs less power than the newer pis, while still having enough computing power to not lag unless pushed hard.

I definitely agree with SSDs. HDDs don’t do well when rotated when running. Boats are less than a stable platform.

I hit the limit of my Raspberry Pi 4. It would periodically crash itself by overheating (Heatsink was hot to the touch) I now use a NUC. It runs excellently, and handles my home automation setup fine. Unless I start doing something extreme, I can see myself overloading it.

One of the less mentioned things with self hosting is running costs. A Pi is extremely cheap to run. A NUC is a bit more, but still well below a full blown PC. Servers can actually pull a significant load, even when idle.