I would recommend trying to find a used one. You will end up with a lot of expense into making one by the time you get all of the hardware and wood. I also don't recomend trying to convert something that was not intended for horses. The type of wagon to look for would be a buckboard. They are usually flat behind the seat.
My Dad and I are in the prossess of building a 4 wheel marathon cart, but Dad has alot of knowledge in all kinds of things. We are going to build the frame out of square steel tubing, and put either expanded metal or aluminum on the parts that should be solid looking. Hydrolic brakes front and back, fifth wheel with brake, etc, etc.