It drills a 1/2 inch hole, so that's 12.7mm
Yes, but I am assuming you mean the K5 not J5. This jig works with the K5 basic as well as Master system. It specifies this in the owner's manual for the K5 system.
For that application, outdoors and heavy wood, I'd definitely use the HD jig & screws.
The HD jig uses a slightly larger diameter drill bit and thus larger headed screws than the other Kreg jigs, so this particular HD jig really only works well with the HD screws (one size only 2-1/2 inch). The blue coated screws are very close but still slightly smaller but they will work in lighter duty applications.
It doesn't come with one. I've used it with the one from my K4 as well as some other (non-Kreg) clamps just fine. Most of the time I'm using it in the K4 so the clamp isn't an issue for me.