My last four cars have been Volvos so i have a very high regard for them, although none of them have had genuine (highly regarded) Volvo engines. I don't remember any of these cars failing MOTs for rust, my current one is now 18 years old, I've had it for over 15 and still no serious rust issues, most of the problems are down to Ford parts during their ownership of the marque, including the Ford/Peugot engine that, at around 80k, started leaking fuel past one or more injectors requiring oil changes approx every 3.5k miles (or probably upwards of 2k on new injectors). Fortunately i only do about 5k miles pa these days so have tolerated this for the last few years and will be selling the vehicle once i get a motorhome.
We sold a narrowboat last year, it had a highly regarded Yanmar (Japanese) engine in it, marinised in the UK by Barrus, built approx 2002 and the engine oil level never dropped a millimetre between annual services!
With regard to the Ducato, I'd be interested from you or anybody else in the approximate hours work involved to cut and replace rusted metal should it ever be required so i can set money aside, for example around a suspension mounting or wheel arch. Whichever motorhome i decided to buy I'm guessing that on the budget i have it will always be a risk. I'm hoping to have a motorhome for three years, use it extensively and have a lot of fun, then reassess, if i still enjoy the experience maybe then i will be able to afford a post-2007 model with a galvanised metal cab.