Small domain setup

I would just like a little bit of help regarding a new domain setup.

I know how to specify the domain name, setup active directory, setup dhcp and all that...but the main thing I guess I am asking about would be licensing?

I recently had a small business ask me about setting up a small domain with about 20-30 users that needed access to files across the domain, aswell as email access.

Could someone please help me understand about licensing, about simulatenous connections and concurrent licensing? Becauses thats where I tend to get really confused.

Also, would someone please be able to recommend the specs of a server that would be suitable for:

File serving,
Active Directory,
Exchange,
Web Proxy