“1. Type of nail gun:
I am going to buy the Bostitch RN46. I’m guessing this is a good choice. It will only be used once and resold.”
I don’t see why you would buy a nailgun. It will only be good for roofing and for the money you spend on it/compressor/coil nails you could have hired an experienced roofer for a couple of days. I roofed daily for 15 years and my right arm never jammed, I still hand nail when I need to jump up on a roof.
“2. Type of fastener:
I heard electro galv is a poor roofing nail. What type of fastener should I purchase (type of metal and coating).”
You need special coil nails if you’re using a gun. Not sure how much variety they offer. I think just about any nail is going to outlast a asphalt roof so no worries there. Hot dipped galv sucks for hand nailing though, the burrs tear your fingers up.
“3. Length of fastener:
I have 1/2 plywood sheathing and I am installing a 50 year architectural shingle. I heard if they nails are too long they can work their way out. I was going to buy 1 1/4in nails. Please give me your opinions on what size nail to use.”
Depends on the decking + shingle thickness. If you’re using a 50 year shingle chances are good you’re looking at either a 1-1/2" or 1-3/4" nail. Check the shingle specs.