ms exchange 2k3 configuration

if the pdc has been named domain.local what would be the best way of including an exchange2k3 server? the server would be sitting on a dmz with nat.
1/ should the exchange server be part of the .local domain,
if so what would the mx record be for the isp dns?
2/ If not should it be configured as a seperate dc and named domain.co.uk?
if so how would you if possible intergrate the AD from .local with the exchange machine?
3/ Is there a better way altogether?