...Is there any other option than Linux for a server OS? Like, practically? I know there are a bunch of others but speaking about everyday classic needs... is there really a choice?
Budget -- I never did mind paying $1000 instead of $150 but it has to be worth it. I don't want to sheepishly buy the latest and greatest, especially after seeing many times in my programming career how a very modest machine (when properly tuned) can be a very decent server that can last you all the way until you need rented dedicated servers in several data centers.
(Example: I gave up on the idea of an OpenVPN server on an RPi 3 after making a research on the expected bandwidth. It would be too small.)
Thanks for the recommendations! Writing them down (not joking).