I am looking into automating the link between the SDP on demand and AD for requesters so that our information is up to date but I got a few queries I would like some opinions\help on. 

If I link it with AD will the existing user database be deleted or duplicated as some of the users information is different in SDP than it is in AD. 

We have a lot of external requesters who email in who are not in AD and would like to know if when I do automate the information to SDP will they be deleted?

Also where do I need to install the provisioning app to be most successful?

Any help or advise will be greatly appreciated 

Many Thanks 

Simon