I don't know exactly what you need beyond just making it manifold and lower polygon. So here is a manifold version of the model (solid) with only 9k polygons. You'll need to subdivide it to make it smoother, but I can do that for you if you don't have a capable software to do it.
https://www.dropbox.com/s/b9zwpa7flxczfah/tinker_lowpoly.OBJ?dl=0