Get a dually long bed of sufficient capacity to handle 25% of the GVWR plus whatever is going into the truck (that includes passengers).
Problem with a gooseneck is it moves the pivot point down which creates a greater lever arm on the trailer frame. Many are not designed for those extra forces. If you look at dedicated gooseneck trailers, they usually have it welded to large I-beams...you won't find anything similar on your typical 5er. It may work for a while but fatigue loading creates a high probability that you eventually have a frame failure.
