I have just changed my turbo charger and I don't get 3500 RPM at the moment. An overhaul of the trubo is for sure not bad, but maybe you won't get your envisaged result.
I thought first that I had a bad diesel or some bugs in the tank. The diesel is cleaned and the tank as well.
With all this I get max RPM 2900. Maybe I have to overhaul the engine specially the injection pump and at last also the injectors.
For your case if you get 2800 rpm so maybe no for sure the turbo charger is working and can't be blocked. Blocked turbo get max rpm 1900 to 2100.
Maybe you get a note when the last big service on the machine was done.
In may case I Need a big service, because my SM was not moved for 3 years before i bought it.
Concerning the Generator, be careful, that you get also a note, that the anodes were changed recently. I used my Generator for about 10 hours and I had to change the pump and the heat exchanger as well, because both were corroded and were leaking.
Have good luck, many pleasant moments on your SM.
Fair winds and also a handsome water under the keel
SM 124, South Brittany