Rob, you may be talking to the wrong crowd for advice on selling an "improved model" because we are a do-it-yourself group. Most of us would probably want the fun of improving it ourselves! That said, the mattress and tongue box would be nice. Lights and an inside shelf (does it have one already?) and maybe a utensil drawer hung on the underside of the counter might not be bad either; but, those would be luxuries on this model. I wouldn't bother with a built-in sink because, if I remember correctly, the back is open to the mattress area. Any leaks or spills and somebody could be sleeping on wet bedding. That outside tap is a nice conversation piece; but, I'm guessing that most people would shy away from that. Ditching that, if you can, is a good idea. ...Just my thoughts.
I had another thought on that tap.... If it can be gravity fed, it might be used for water. Put the jug on the roof and a hose from the jug to the tap and voila! Is that the way it was originally used on this trailer?