Another method would be to use AutoText. The AT entries would be stored in separate global templates, one for each company, on that company's system loaded as Global Templates. Your front matter would have AutoText fields which would get their content from the global template loaded on the computer. The AutoText for this is not stored in your document's template because you want it to change based upon location.

I used this method to design letterhead for a law firm with over 50 offices that had different personnel and addresses on the letterhead depending on the location.
Automated Boilerplate Using Microsoft Word

The choice of using AutoText fields vs. IncludeText fields is yours. Either one is about as difficult and AFAIK they will work about equally well. I've been using AutoText for so long that I seldom use IncludeText fields. So long as things are set up properly, they are pretty much transparent to the end user.

And yes, you probably could use IF fields, but that would be doing it the hard way and would require a fair amount of field coding.