We've had a pretty deep discussion of some of the van powertrain details in this thread. But going back to your original post about wanting a basic travel van with simple bed conversion in back, I'm going to suggest you ask the van conversion company which vehicle chassis they suggest as most appropriate.
To the best of my knowledge only Sportsmobile and Van Specialties (Oregon) have experience converting all three Ford Transit, Mercedes Sprinter, and Ram Promaster units into RVs and camping vans. Sportsmobile tends to build complete campers with built-in kitchen, cabinets, and beds. Van Specialties converts a wide range of units all the way from full RVs to work vans and relatively simple campers like you describe.
I know you're concerned about vehicle reliability. But Consumer Reports points out that the least reliable vehicle from their 2014 survey was more reliable than the top finishers from the mid 90s. So although problems can occur you'll most likely have good reliability with a new van.
Safety might be a more significant concern given that your kids will be riding in the back. New vans have better stability control, tire pressure monitoring, airbags, and controlled crush zones than your older Vanagon. And for the convertible seat/bed at the back you'll want to make sure it's got full seat belts and safety anchors. Van Specialties seems like a good resource for those questions also.
