You don't have the option of using pins with buckskin, so have some painter's tape handy. I would use a leather wedge needle, AND a walking foot. Buckskin will stretch, a LOT. Painter's tape will come in handy there too...cover the back of your leather with the tape as a stabilizer, easy peel off when you're done with your seam.
I'd stay away from the decorative stitches, because like kountrykreation said, leather is not forgiving. Once you make a hole, it's there forever. I would think that the multiple perforations would weaken the leather, making it more likely that your zipper would pull out over time.
I don't think it matters which way your needle faces, but I would be tempted to use a "front to back" threader, with a buttonhole twist thread. If you use the buttonhole twist thread in both the top and bobbin, you will most likely have to adjust your bobbin tension.