The purpose is to generate proposal documents that can manually be edited in Word after the fact, but before sending them out to the customers.
Much proposal content would be drawn from existing HTML website content (backing CMS) and also some custom (non-HTML) injection for certain scenarios. Of course the conditional logic could go into server-side ASP.NET to vary the content appropriately.
I’m open to 3rd-party tools if raw manipulation of the Word API is arduous. In fact a good 3rd party tool might be the answer.
Use the Aspose Words component for .Net.
Aspose Words Component Link
The component natively understands the Microsoft Word file format without having to install any Microsoft Office products on your application environment. You can then start from an existing word template or programatically build up an entire Microsoft Word document from scratch. The Word object model then allows you to export to doc / docx etc and save as a native Word file to wherever you required.
They have plenty of demos set up on their website.