This company in the UK specializes in turbocharging na engines. http://www.tb-turbo.co.uk/about.htm
They do a lot of Mercedes motorhomes. I talked to a gentleman who had his MB motorhome turboed and he was very happy with it. A lot of people on this list downplay the idea but I don't think anyone here has tried it and can provide empirical evidence. I've seen a number of turboed 240d on Ebay and some of them have high mileage so I'm guessing that they do last.
I'd ask that company for specific data on the longevity of their turbocharged engines.