You don’t need to re-case them. I had three old thinkpads as servers a few years ago. Took the batteries out and connected them to Ethernet switch. Built in KVM. Job done!
I gave up on worrying about those details - just installed ProxMox, set it on a shelf in the basement, moved on. Works great for the tasks I'm running.
I was even thinking about how much mileage could one get from a rack of old laptop logic boards. Gaming laptops sold for parts are much, much cheaper than "real" server hardware. It appeals to me because of the reuse aspect as well.
Holds true regardless of your workload :) Imo sounds like premature optimization.
I had an old dell laptop that had a double-high pcmcia port and I inserted two xircom realport LAN cards and had a very nice router with three NICs and no dongles.
But these days there are often controls in place to cut off power if things look bad (battery getting too hot, mainly) so things are more likely to die on their own rather than setting fire to themselves and anything near-by.