in response to ur questions
1) Can I use dyndns to setup my server at home, if not I'll ask one of my colleagues to give me a public IP
yes as long as dyndns points to your public ip at home
2) What ports need to be forwarded from my router. Currently I can only pick up port 80, but would imagine port 25 and 110 also needs to be opened for mail access
Ports 80 (HTTP), 8080 (ISPConfig Admin Access), 21 (FTP), 25 (SMTP), 110 (POP3) & 443 (SSL (optional))
3) When I create a website for a client,
a) do I create a FTP user for this client to access my server?
yes, under Sites>FTP-User
b) where does the website files go?
When logged into the FTP, files go into the /web/folder
hope this helps