Everything except the number of Wifi users: theoretically doable.
GLinet sells devices with mobile broadband (3G/4G/LTE etc) and OpenWRT, but I've never used mobile broadband or those particular models so I can't comment further.
For even 50 wifi users you will want several wireless access points. Exact count depends on what link speed (Mbit/s) you need them to be able to achieve and the max limits of your particular models of wireless AP before people start getting an unreliable link (disconnects, etc). Lookup "estimating number of access points" on the web, there will be lots of sources giving you approximate numbers.
If it's a product for other people to run unsupervised (ie without you around): best thing to do is try it. Buy a few different pieces of hardware, test with lots of users, see what goes wrong. Things that you never expect to go wrong will still go wrong, especially when money is involved.
Probably not what you want to hear, but this has been done commercially in numerous proprietary projects using openNDS, and available, I think, to purchase as a complete system from at least some of those companies.
But still, openNDS is open source so you are welcome to develop your own version of what has become known as "Museum Info Portal" or similar.
You will probably get some hints if you open an issue at openNDS on Github at:
it's probably working just fine, but it can't be flashed.
but it also depends on what version of the loader the devices ship with, it might be old enough
from factory, but gets replaced when/if you update the official fw.
only making sure you're aware of the risk, don't own the device myself, but happened to see the thread.