All in one box ! Is it a good solution ?

Hello, One of my friend ask me several questions concerning the security of a small company. He'll get in few days an ADSL connection. For some costs purposes (I think) this connection will shared with another company. The goal is to get a Mail server, Web access, ...and share all this with his LAN I've suggested to set a Firewall like ipchains, first to be realy separated from the network of the other compagny; Does it make sense to set the Mail server on the same machine ? How does this will be setted because I think all Mail clients will be Macs ? Thanks for your help Bests Pascal MiQUET