When my Idler tore up the top-shaft tore up too... I bought a titanium top-shaft, steel idler, and an annodized aluminum diff ring gear. When I put it back together I left the stock plastic diff ring gear in and kept the metal one as spare. I haven't needed it yet.

Just run what you have stock, if it tears up, then upgrade, but not before. Like I said, my ST-15 had a tougher tranny and never needed the parts... maybe yours won't either.