Whether the E150 is the wrong van is really based on what you want to do with it. You might need to go with a lighter build to stay within GVWR. The 4.6 is another reason to not go too heavy on the build. On the other hand, I believe the newer E-series E150s are built with the same frames as the E250.
Many people/converters start with a E250/350 or 2500/3500 due to the weight of a build.
Someone with more E-series knowledge can address that.
2008 GMC 3500 Quigley Weldtec 4x4 Savana SMB