They show both styles of carb for your motor. So you probably have the right one. After your compression and spark test make sure the right gaskets are used. That carb kit has both sets of gaskets in it. Easy to screw them up.
That plug should be available everywhere. They used for like 20+ yrs. I would stick with Champions the Ngks can do weird things. Like foul prematurely, cause misfires just weird things.
Well the fallacy of not charging a dead battery with your motor is BS. I have seen that happen on Mercs before replace all the connectors then test the charging system.