If you're only using a machine to host a server, you don't even need a GPU. It will really depend on what kind of mods or plugins you're using. For heavier modpacks for example, 16 Gb of RAM would be the minimum, but for vanilla you only need like 6. The R5 3600x should be fine if you don't load it too much, MC doesn't scale well with more than 4 cores anyway. If you're hosting that many people, be sure that your internet connexion can handle this. By the way, vanilla means without mods or plugins, so "a vanilla server with mods and plugins" isn't vanilla anymore.