Following on David's comments: if I, as a soon-to-be retiree, decided that I'd become a camera repairman focusing on one or two particular makes, what do you think they should be, putting aside Leica and Hasselblad?
There's a good reason to put aside Hasselblad - the buy-in to become a Hasselblad repair tech is quite high, as they require some specialized, proprietary tools to fully service the cameras that cost thousands of dollars apiece. I'd look at Rolleis - as far as I know they don't require expensive proprietary tools, and the market for servicing will bear the price you want to charge. Nikon would be the other brand - they have such a cult following (especially the earlier cameras like the F/F2/F3), and there are so many of them out there that you'll have plenty of demand for your services. Canon would be a third good option, for similar reasons to Nikon, although there would be less demand I think due to the general abandonment of the FD mount not only by Canon but by their customers as well.