I don't know an exact number, but I've seen others say they paid 1500-2500 for their studs. My opinion on them is this, don't fix if it ain't broke
I like my ATS exhaust, but it was waaay overpriced, sounds like a big block gasser when I'm WOT
I don't know anything about the high idle mod. Your truck will idle up on its own when its cold, so maybe its for a PTO or something???
IMO, leave your intake stock, and are one of the best filtering systems on the market, and flows enough air for around 500hp.
Zoodad isn't bad for the engine, its a good thing because it will lower your EGT
My neighbor has those wheels on his truck, nice looking, but I'd hate to pay for tires on those 20's. You'll get some other opinions on your questions, but that's what I have for you (there are a lot of good helpful people on this site). LMK if you have anymore.