Ubuntu Networking :: Sharing Internet Access And Local Files/server

i would like to setup one of my old pcs as a file server and internet gateway; we are living in a large building shared with 40 others. the ubuntu box would be the one connected to the internet via ethernet and sharing connexion via wifi. i haven't started yet - presently, i am doing the ground work and reading before to start i understand sharing the internet is relatively simple and can be done from the GUI

What we would like is slightly like BT Fon or BT Open Zone in the UK: you can hook on a free network but in order to access outside (internet: email, web, ftp, etc) you need to login login would help us monitor fair usage. I imagine something with username and password for each user would do: as we are a few in the same building not everyone is actually paying for the connection and we don't want to end up with rather large excess bills. So the ones who are paying access both files and internet; those who dont just have access to the files on the local server.

Do i need Ubuntu server to set this up? What hardware would be ideal - given we are all far from rich but willing to have a nice setup

It would be great if you could share some knowledge around the topic and eventually provide some tutorial; also any heads-up on the hardware side would be great! (signal booster, etc - there's 3 floors and 3 buildings)