Tony - there is no such thing as a major or minor rebuild of these things. Whatever has to be done, the booster has to be dissembled and that is quite tricky as it contains a large and powerful spring. Then the thing has to be cleaned and a replacement bucket and seals have to be fitted. Don't bother with the old bucket that could be at least 65-70 years old, these are life saving brake componants you are talking about. When I obtain the other contingencies I talked about I'll see if there is a spare one that I could get rebuilt.
With the master and slave cylinders any good brake place can clean, re-hone or re-line them along with putting all new componants in, i.e. pistons, seals and boots. You don't need to buy from overseas. Doing it all locally also gives you a comeback if anything untoward happens. I would also be wary of using your old brake linings, get them professionally replaced. No matter what vehicle I restore I always get the whole brake system professionally done. Brake lines are easy if you have templates or else if you have them, just a good flush is all that is needed.
Bob