I think it depends on your server. My partner has a pretty good machine (cpu AMD Ruben 5600x, gpu 3070, 16gb RAM). The RAM is the limiting factor when playing the game and running it on the same machine. We could do it on his machine for the two of us and three other friends and minimal mods but he had to make sure to keep all other background processes closed (definitely no Chrome!). It wasn’t a long term server though and we just ended up switching to a rented one.

And yes, you can copy over your single player save and turn it into a server. I’m not sure if anyone other than the host character would copy over though in a co-op save since the friends are dependent on the host and aren’t “real” characters exactly but I’ve not tried a co-op save, just standard single player.