Mike raised some good points. I'd like to add my own observations.
Probably the most important part about choosing a B is the floor plan. There is limited space so you have to choose a floor plan that suits the things that are most important to you. Some people want an elaborate bath with shower and lots of black tank capacity, others choose a Porta-Potti and primarily use campground facilities, etc. etc. Each feature you choose cuts into space for other features.
Common chassis include the Chevy/GMC units and the Sprinter diesel units. Dodge has a new unit based on the Fiat van and Ford is coming out with Transit models.
Check local dealers and RV shows to see how each model fits your needs. Are the driving seats comfortable? Does the unit make you feel claustrophobic? Lie down on the beds, sit in the dining area, pretend you are using the head and showers, see what its really like to prepare food. Can you kneel in front of the head in case you become ill?
Currently all of the companies that make B vans have a pretty good reputation for quality work, so there isn't much concern there.
There are a few companies that make custom or semi-custom units. Advanced RV in Ohio makes very high end units and Sportsmobile in Texas, Indiana, and California make affordable, but still high quality products. The Sportsmobile website (www.sportsmobile.com) is a good source of information about all the possible features and floorplans available for B's and I recommend it as a source of information.
Advanced RV Sprinter