Ralkage I'm going to counter some of this:
Ralkage you can only imagine the chaos of 12-200 users running Composer commands at the same time
Hundreds of users running commands at the same time isn't really going to happen under usual circumstances. An application is installed or updated occassionally.
Ralkage SSH, on the other hand, is typically disabled or if you're lucky enough to have "jailed" access is typically due to security concerns and for the fact that users can do malicious things such as finding a way to hack into another remote directory if more than one user is sharing the same resources and server instance.
This is true, but there have been decent solutions for this like cloudlinux.
Ralkage To be frank, I'd just go with VPS as they are relatively inexpensive nowadays and have a range of tutorials for you to get things up and running (DigitalOcean has docs on how to setup LAMP and LEMP respectfully).
In case you do attempt to set up a virtual server according to tutorials, remember that setting it up doesn't make it secure. You actually have to harden the machine, keep it up to date and do decent monitoring to prevent any loss of data, data corruption or even a hijacked server. There are so many things to consider that running a virtual machine for anything but pleasure without decent experience is better left to professionals.
Grabsky I've read that NGINX is a way faster than Apache2. Is that a fact? What do you guys prefer me to use for Flarum?
Nginx is faster, but it is more detached in modularity, meaning you have more to configure and tune. Using php-fpm and their pools is a great example. Apache instead makes php work immediately when you installed it's module.
This forum runs on nginx by the way. But Flarum really doesn't mind either of them.