How to install Exchange 2010 so it is as independent as possible on our network

We are going through a company merger and people want to do things differently!

Current situation:

• 2 DCs.
• 1 Exchange Server on 1 of the DCs (not SBS).The other half of the email is handled on a hosted Exchange server.
• Half the PCs are joined to one domain and half are not joined to any domain (the ones using hosted Exchange).

Currently desired situation:

• 0 DCs (I know this isn't possible when using Exchange).
• Everyone's PC not joined to a domain
• 1 Exchange server on the network that everyone can set up their Outlook to log into with both former domains MX records pointing at it.

I have a clean box to install 2008R2 and Exchange 2010 on. My current thought is to set up the Exchange as it's own domain and just set up the users on it and have them log in. Almost like setting up my own hosted exchange but on-site! This means the DCs can be ignored or demoted as desired and Exchange can sit there doing it's thing without being concerned what happens elsewhere. Is this the best option or are there gotchas that will kill it dead or better ways of doing things? We are a small company (less than 15 users) so convenience and flexibility rather than super-slick administration is more important.