I personally would stick to the W series Workhorse chassis rather than the lighter P series Workhorse chassis. This will avoid the autopark brake which has had issues and will get you much stronger brakes with the W20-22-24 chassis. It will also get you the Allison transmission, instead of the GM transmission on the lighter P series chassis.
The Ford chassis have had some handling issues that can be corrected and the best you can do is get a Ford transmission, which Ford claims is almost as good as an Allison transmission.
In reality, you were given the best advice which is to find a floor plan that works and any of the chassis will get you to the top of the mountain and down the other side.
