There are a lot of posts about this subject. I'd look around previous posts here, and check out Sprinter Source for more info.
JMHO, but I consider them very reliable... but if/when they do break, you do pay Mercedes prices for parts/service, and finding not just a Mercedes (or Freightliner) dealer, but one that services Sprinters can be difficult for non-urban areas. You also will be paying a $15,000-30,000 price premium for the Sprinter chassis as well.
The V6 diesel, the OM642, is a decent engine. It has been around for a long time. The four-banger is OK, but it sacrifices pickup for MPG.