I Used to have an old U-Haul truck, that was converted into a light duty dump. The hardest part about this conversion, is finding a body. Your options are:
Find a used body, sometimes hard to do because of the long frame lenght.
Buy a new body.
Shorten the fram and buy a used body to fit, or brand new body.
If you can find a truck in good condition, and the price is right it isn't always a bad idea.
However i know a guy that works at a ford dealership, that U-Haul uses to service their trucks. He says they bring in trucks with blow motors, trans, rear ends, ect all painted orange. Then they have the Ford guy through it in, Uhaul maintains the truck, only they generally have 500,000 on it when they sell them, i have heard. The old U Haul GMC i bought had like 300,000 K on it when they sold it, i put another 100,000 on it, the end result was i was able to make out ok, because the truck had a used body. However if ot had a new body, i may have lost money.