My organisation is also looking to tackle a problem similar to this and I can share with you some of the things I've found. We're not doing it yet, still in the design stage so take the following with that in mind.
Your situation also depends on the required format at the mailing house - at the moment they must be accepting Word 97 documents, could they also accept XML, DOCX, PDF or something else?
There's a commercial product called Intelledox that lets you use Word 2003 (possibly as early as Word 2000?) as a template designer and then assemble documents from data sources which can be databases, web services etc. Looks like a pretty interesting product, depends on your budget though. This would probably rate highly on your criteria for end user template design.
You could look at the Open XML SDK which allows you to generate DOCX (Word 2007) files programmatically. Your business users would generate a template in Word and supply it to a developer who would then likely create an XSLT. This XSLT is then used at run time to programmatically replace the contents of the base Word document template. This doesn't require Word on the server, just the .NET DLLs that are part of the SDK. Downside to this is it requires a bit of developer processing each time a new template is generated.
I've also considered the reporting approach too - we use SQL Server Reporting Services and that could be used to generate the merged document. It supports Word, Excel, PDF and other export formats. The downside to this is that it also usually requires a developer to design the report. You could deploy the SQL Server Reporting Services Report Builder which is intended for power users to generate their own reports. If you consider this approach make sure it's SQL Server 2008 (Report Builder 2.0), apparently Report Builder v1.0 is pretty poor.
If the mailing house accepts XML you could supply the mailing house with an XSLT file and just send the data, rather than the assembled document. Not sure if the print house you use accepts that, or if you need to keep a copy of the completed document internally for auditing purposes.
Lastly it might be worth a look at the XPS format. Not sure how the templates for this would be design, it might be programming-heavy too.
Hope this helps with your thinking anyway!