Welcome to ALR.
The components you have (lock, stock, and barrel) comprise the bulk of the parts expense. If you are a fan of German silver, that's great. As I recall, it did not really come into use until the 1840s or so.
A Golden Age (1800-25?) rifle would have had brass buttplate, triggerguard, and thimbles. Good news is that those components (particularly sandcast BP/TG) are pretty cheap. A patchbox is a little bit of a challenge, but not rocket science and you could make one that you like from brass sheet. I'd research the style rifle I would like that lives in that stock and get brass castings to build it.
And you can sell the parts you don't use here.
Looks like a great project.
Larry Luck