To create a standalone co-op server for ARMA 3, just check out the detailed tutorials on YouTube or buy the pre-made server you mentioned. I've been building my own cooperative server for Arma for a long time now, so I can't help you, but I think you'll find at least some tips on this forum. I haven't played ARMA 3 at all for a long time, because all my friends I played with abandoned the games. Nowadays I can sometimes go to some CS:GO server and play there. And when I get some skin, I just sell it on this market - https://skincashier.com. Eh, how I wish I could go back to those days...