Welcome to the forum and congrats on your guy (btw, we LOVE pics!!!)!! :):)
A king betta is a betta that can get quite large, possibly 4 inches-ish. It is good that you got a 10 gallon because they need more room than the average betta.
Sounds like you got most everything. The only things I would recommend at this point are these:
A water test kit. You will need to monitor the ammonia, nitrite and nitrate levels in your tank to make sure they don't get too high while your tank cycles (check out our sticky about cycling here
A siphon and possibly a bucket to do water changes.
Gravel for the tank.
Not sure what kind of fake plants you bought, but plastic has been known to tear their fins. Just keep an eye out for that. You can run pantyhose over the plants and decorations to test them. If the nylons snag, then your fish's tail will snag.
Let us know if you have any more questions!
EDIT: You may also want to get Aquarium Salt. It is a good medication for many illness bettas may have.