Best Way to do Mail Merge

We have a requirement to develop a document generation capability in an
asp.net application. We will need a database of many different documents
where we can call methods that accept variable data that customizes the
document. The majority of the documents will go out as emails.

We could roll our own with html documents but there must be a better way. I
have thought of using the capabilities of MSWord and looked into VSTO but
this appears to be a platform for displaying database data in office
documents on the desktop. We are looking at mail merge functionality on the
server.

This post is to try to get "best approach" guidance.
thanks,
T