I'm glad you are shooting PPC. It is possibly the greatest test of handgun fundamentals around. The questions you have are very good ones and I hope you get good answers via this website. My opinion is that the 6" gun would be the best option because of the longer sight radius. The 6" gun will also help sight reaquisition on the rapid fire stages. Even if you leave the stock sights, you would really benefit from blackening the sights with either a sight black spray or a carbide smoker. I am a little worried about shooting single action. I've shot PPC all over the place with alot of different people and very very few if any of the top shooters shoot single action. Just something to think about. Buy more speedloaders if you can and try the safariland comp III type, they are cool.
The course and the ppc rule book can found at nra.org
The b-27 is the target used but I don't know anything about which reduction targets to use.
Get the local number for your Texas Highway Patrol and find an old crusty trooper named Odie Hitt. He is a highmaster and has been shooting forever. He's really not that old and not that crusty. Well maybe he is a little crusty, but he's definitely one of the good guys. He may be able to help you find matches close to you. He knows everybody.
Keep shooting and keep that double action trigger coming to the rear without stopping or hesitating. If you can do that everything will get better, I promise.