My thoughts (in no particular order):
1) What's your budget?
2) The boxes are flimsy and often the tops are not appropriate and they have to be modified in the front / rear.
3) A prefabbed box looks good and can be made strong, (adding structural members, spray in insulation, etc)
4) You can buy a used coach cheaper than you can build one... (unless you have good fab/labor resources and are very good at purchasing)
5) If you want something very specific and don't want to/won't compromise you'll have to build it yourself
6) You can build it better/stronger than you can buy it... see #1, it will cost more.
7) Buying the truck, building the box, installing windows/doors, HVAC, plumbing, etc is only half of the cost.... the other half, which really adds up, is in the small stuff that's hard to think about when you spec the build.
8) Your time is the same as the cost...Stretching the truck and buying/building and installing the box is the fast part. It takes a tremendous amount of time messing with little details and figuring out how each of the components will work with or affect the rest of the design.
9) Figure out how much it will cost and how long it will take, then multiply it by 3! On the other hand...perhaps you're a better estimator than I am :-)