Lucy,
In years gone by I used to be a big fan of diesel equipment. Things have gotten so complicated and expensive, I doubt the efficacy. I've got a large caterpillar c7 powered diesel motorhome, and had duramax Chevy trucks. Those duramax powered trucks were solid, the Allison transmissions lasted 200k +.
Best thing you could do is look on the expoditer's sites for the van chassis you are interested in. Many problems happen as described with the Sprinter chassis. Expeditors haul small cargo long distances and rack up many miles on their vans.
I've got a gas class b also. I think as you research you will find most gas chassis capable of 300k miles if properly maintained, same as the diesels and maybe a little better than the sprinter. Added bonus? Easier to get service anywhere for a Chevy or Ford product, parts are plentiful, and aftermarket providers more numerous. Keep in mind any class b will be pretty hammered in general after 200 k so your research may be moot.