A 24 port 100 mpbs switch (any MAKE) or 2-3 (8 port) hubs for seperate deparments.
A DSL router for your internet connection.
Personally i am using 1 IBM x series 206 server (P4-3.0 Ghz, 1GB RAM & 76 GB HD) for my main samba file & print server & few normal P4 boxes for Proxy & backup server.
For mailing solution now it depends on your company.
Get some enterprise edition distro cds. I use RHEL4.
I am also having a setup of around 30-40 computers in my company.
Reasons for *NIX..

Low hardware costing.
More secure networks.
Lesser virus/spyware threats.
& reasons to visit
LQ
And last & most important things, Get yourself trained or get used to of things you are going to implement in real live network.
Thanks
..amit..