For the engine I have found a place in the UK that rebuilds them, they seem very good but he has difficulty finding the 100's in good condition. I also see that for the type of old man flying I want to do the 80 may offer a little more comfort. That said the 100 in the mountains we have round here would be a reassuring advantage. I will just have to see what is available at the time.
I fully agree with what you say about the corrosion protection, it drives me nuts reading all the contradictory opinions and advise. It may well be that a compromise is the best solution, protect all closed and hidden sections and leave the exposed parts with a "wax oil " type spray if needed.