ok, so it's not really made by GW, but my friend and I converted what used to be a toy monster truck into this beast which we use as a battlewagon. I think it turned out pretty well.
This is actually quite awesome. The only thing that I think needs work are the wheels, they still give the feel of a toy truck. You could try to add some mud, but I think you should make them look like they were solid steel (it is a battlewagon, after all).
Overall a great job, though.