I've used several laptops as file servers in hot apartments without AC and haven't had issues. The trick is to make sure you're not running a bunch of bloat you don't need (which is the root cause of 99% of "my old computer became slow") Slap something like debian on the laptop and don't even bother installing a desktop environment. The load of an NFS server sitting there idle most of the time and a few other system services doing basically nothing is negligible, any laptop should be able to handle it in 40 C weather. If not then you probably have some dust issues, so blow it out and see if the issue resolves itself. Keep the screen partially open (you can turn it off though) to help with ventilation.

I've never had any laptop die while operating like this. I only replace them when I replace my primary use laptop with something new, demoting my last primary use laptop to the server role.