The biggest thing is that the Webasto and Aqua-Hot are run on diesel and are made for mobile use. Anything that heats water should do just fine, but keep in mind that electric will consume lots of electricity and propane will consume lots of propane. Here are some other options...http://www.alsheating.com/OM-22.htm
This one will run on diesel. It is like a monitor so will not heat your water, but they are very efficient. The vent pipe would have to exit through the side of the bus, but if I were planning on fulltiming in cold weather, I could find a way to live with that.http://www.tanklesswaterheaters.ca/oilmiser.html
This is a tankless water heater. I once measured and figured it would just fit (height wise) in the MC9 bay. Venting would be tricky and once you got the furnace and a lifter pump, you'd probably have close to 2K in it. For that, I'd throw a few more buck at it and go Aqua-Hot or Webasto.
I really like the idea of the Monitor style heaters. I had two of them on my boat when I was living aboard year round in Maine.