I have rented two motorhomes in Germany and the coaches were made in Germany and then placed on either a Fiat or ford chassis. Both diesels and the fiat was better.

IMHO, from just these two, which were a couple years old at the time in 2003 and 2010... I would say that the coachwork is not better than US made. It is about the level of a medium quality class "C" middle model in fit, finish and function.

You are faced with weight and power. Because the smaller Sprinter-like models, class"B" or small "C" have less power, they need to shave weight. The interior suffers as a result.

Maybe you can rent locally a sprinter model and see what you like or dislike. I have rented in the states and I have owned Class "A" models. I would not try to build out a base sprinter to a motorhome. It will cost more, do less and be less than enjoyable to drive.
Just my two cents
HTH