In part it depends on whether or not you will be reusing the information, that is using the same letterhead on a number of occasions. If so, consider keeping the information in a series of global templates that hold the information as AutoText. You could use the UserForm to store the information in a separate template. To use the different information you would load and unload the AutoText holding templates to switch it out.

Are you doing a mailmerge of additional information or are you simply typing letters? That is, are you sending out form letters with this information? If not, mailmerge would still work fine, I would expect. I have hundreds of mailmerge forms and letters that I use with a database of clients. I usually only do one at a time. I will do different forms on different days for different clients. The idea is to have one repository of the variable information and use it as appropriate. In my case, the forms all give me a rough draft that I adapt for the client's needs.

In your case, it could give you the your letterhead.

With either AutoText or a mailmerge you could have different text and even logos in the same positions in your letterhead format.

Here are some additional letterhead resources.
Tutorials
These are downloadable samples showing how different features work in practice.