your gonna be better off having it built in separate stages go buy the truck and take it to a place to have the tank and pumps put on buying it as a package sometimes costs more.
good choice on the cabover tho you can find some GMC's at a good deal i was poking a stick at this idea for a while and GMC has a 5500 weight class truck that is a cab over.
used or new truck?
how many gallon tank?
i just use a skid setup i slide into the back of one of my trucks.