I don't do merges with Outlook, but everything I've seen says to start them
from Outlook rather than Word. You use the term "master doc." There is a
dangerous feature in Word called "Master Documents." I assume that we are
not talking about this. The term I've seen used for what I think you are
talking about is "primary merge document." Another key term is "template"
which refers to a kind of Word document that is intended to be used as the
basis for other documents. You can, and probably should, have a template
that you use to start your merges.
http://addbalance.com/usersguide/templates.htm

If I were attempting your project, I would start with
http://www.gmayor.com/mailmerge_from_outlook.htm. If that wasn't enough
help, I would explore Graham Mayor's site for other ideas and then look at
Cindy Meister's site. They seem to have the most comprehensive information
on mailmerge.