There are probably many ways of accomplishing such a task. However, one that could work for you would be to use some VBA behind the doc.
You would need to create a form which would provide a list of clients drawn from the database. The forms available via the VBA/macro environment behind word are not as powerful as Access forms but you can create form with a combo control and populate it with data from a recordset.
The form and database query would be activated using a custom toolbar button after the template doc is opened.
Then all you need is an On-Click event which creates another recordset based on the selected client and inserts the contents of the recordset (ie address details) into the doc. Finally the user would simply finish with a "save as".
This describes the broad methodology I have used to populate an excel spreadsheet (data and graphs) - I'm sure the principles can be applied to word.
Hope this helps.