Victory Point Games sells 5 expansions for the base game. It also lists each previous expansion as a prerequisite for given expansion. Is that strictly true?
So if I want to add arquebuses to the game I need to buy 3 expansions? (Strange Ordnance, Hell's Horsemen, and Elephants At War).
You likely won't have all of the bits for some of the scenarios if you don't get the previous expansions. I never bought #3, but Elephants come in expansion #1, and there's at least one scenario in Expansion 2 that uses them.
If you just want to build your own scenarios, my guess is you can just get the arquebuses and skip the previous expansions.