If u are referring to the mac mini server, i think it would work nicely for the situation that you describe. i have two in my office (one will go to a data center in America soon). The server is full fledged: web, ftp, etc. and u can have lots of users connecting from other types of machines. I don't have monitors or keyboards attached to my minis (headless), I just use Apple's remote desktop to connect to them from where ever I am - even if I am away from the LAN it works! They will work with any router and come with two harddrive slots, so the options are there for something much better than time capsule. Highly recommended device, although I have little faith in Apple maintaining the software for them (see discussions on SL vs Lion vs ML server app). Cheers!
If you did chose to use a mac mini as your data server; you could attach an external hard drive to the server and use time machine to backup the files on the server. GIving you the ablity to have a backup of the server if it dies AND have some level of historical backup of data.
a mac mini with file sharing means you can have more then one area to drop all the files. That can be useful if you don't want all your staff having access to every thing.
Mac OS Server offers windows and mac file sharing, so pc can connect to the server localy.
Some thing to keep in mind, if the some one is using the 'server' as their computer.. What happens when they need to reboot? How dose that effect other users? What happens if you need to work on the server, but they need to work on some thing else?
To use the mac mini as file server, that other users can access over the internet may require some addional work. If your internet connection isn't static ip , then you need dyDNS, or to upgrade to a static ip internet connection. that way user can conistently reach your internet connection.
If your internet connection has a built in router, then the router has to be configured to allow traffic from the intenret to reach your mac mini server.
If you want to be able to use Some of the feature Mac OS X Server offers over the internet you may find they you need statip ip internet, DNS name registration, and a SSL Cert.