I had a set of three capture templates for memos, a number of years ago. It was very convenient. A letter is usually pretty much the same thing. I don't even know whether I have the original templates around anymore; it should be pretty easy to implement. IMHO, easier than dealing with all of the header materials you are describing. I think, anyway.
Two files were invoked: a header and a tail. Then there were capture template interactively filled addressee and whatever else. One had a letterhead. It was, admittedly, a kludge. A kludge that worked and saved a lot of time. For memos it's maybe more sensible, as they are quick one-offs, for the most part.
Alan Davis