The most important thing to understand while you consider your options is this: Except for resource usage, using multisite is like using completely separate Discourse instances. No data is shared. From the outside, the setup will be (almost) indistinguishable from you just having one server per Discourse installation, each simply set up according to the normal install guide.
The only – but very important – advantage of multisite is resource usage: For the system requirements, you can consider all instances as if they were one large instance, which heavily reduces RAM requirements 