Exchange Merge\Migration

Hi
I have a couple of questions about an exchange migration project. Client just acquired a new company that also has exchange, and they would like to migrate them to their existing exchange.
1.what is the best why to go about this?
Migrate AD domain (users computers… using ADMT) then what about exchange mailboxes?
2. what if want to leave the domain as is, but migrate mailboxes and decommission the exchange server, is it feasible?
3.Anybody using RPC over HTTP? any issues?
Thanks.