Some direct answers to your questions.
1. Waldron's exhaust carries everything you'll need to make your system correct. Check out waldronexhaust.com
2. According to the 1951 parts book, the hinges should use 3/8x16x3/4" hex head bolts. That said, every car I've had used round head Phillips style screws. Attached is a picture of what's on my 53. I checked the 52/53 parts list and it does not mention different screw/bolts so I assume the same were used from 1951 thru 1955. I would go with the screws.
3. If you are a KFOCI member, check the monthly bulletin as a member sells the correct plug wires for your car.
4. KF used two styles of mirror. One style clamped to the door edge ahead of the vent window. The second style mounted to the door below the upper trim and at about the leading edge of the vent window. They are not available today and it's been said they are the same as concurrent Hudson. Truthfully, as long as they look period correct, there isn't an AACA judge who will know the difference. Been there, done that. This thread discusses the mirrors used ->
http://kfclub.com/forum/index.php/topic,11612.msg52802.html#msg528025. The club is having the correct fuel tank reproduced. If you don't want to swap now, buy one for later.
6. KF used 6.70 x15 tires on their full size cars but 7.10x15 is accepted as well. Goodyear was the tire brand used and the whitewall width should be 4 1/4".
AACA judges do a good job of performing an overall evaluation of one's car. Some of what they look for are wiring that's not correct, nuts and bolts that aren't right or are out of place, overall condition, paint color and correctness, interior condition and if it's entered into a stock class, how closely it resembles what came from the factory. Keep in mind that few AACA judges own a car like that of what they're judging so they have to use their judgement as what is and is not correct.
Good luck with your car - I'll be attending the meet so we'll see you there!