The mulching blade setup is a little much for that engine but it's how I have always run my Masters series Honda. You just need to make sure you let it warm up a couple of minutes before trying to engage the blades and not be mowing tall lush grass.
The mulch kit includes a mulching plug, upper mulching blade and longer blade bolts. The spring washers get reused, assuming they're still in good shape. The blade bracket is not used for the mulching setup. I got mine from my dealer at the time of purchase of my mower.
Yes, it's a nice mower but I don't use mine much being as I mow commercially and mostly use larger mowers, so even though it's a dozen plus years old I can't really speak about longevity.
Edit: It appears I have a different Masters series model. Mine is the HR215K1HMA. Mulch kit should be the same though. Item number for the kit on my invoice is 018009 but I didn't find any matches for that in an online search.