Active Directory Query

Hi,
I am planning to deploy a new AD server(on Win 2008)for one of my Application accross all the Environments(Prod/DR, UAT, Dev. Test.)
I have few questions regarding the deployment, I am looking for experts people advice before deploying the same.
Explanation:-
1)For my organization i want to keep the Forest domain with me.
2)There are couple of vendor organizations also, which i intend to put in some different domain.

what are the different considerations which i need to take care of, for achieving the best industry standard develpoment.
3)