You can still buy NT4 and Exchange 5.5.
It depends entirely on you which you go for.
80 people is easy. Single server, Pentium II 400 will cope fine, 256MB RAM is ok, more if poss. Suggest at least a 4GB data disk for the database (2GB for the data and another 2GB for expansion).
100Mb netcard at least. TCPIP as the only protocol (unless you are running Macs there), DNS to help outgoing mail, 2 net cards for security (one on the Net, one internal) unless you have a firewall. It should be a domain controller.
NT service pack 6a and Exchange 5.5 SP4 or Windows 2000 SP1 and Exchange 2000 (has this had a SP yet?). Your choice.