I didn't buy any 45's i just cut up a 90, although i didn't check, its probably cheaper. I used 12 90's for the header, and i'm going to use maybe 1 more to make the wastegate tube coming off the collector.
ive never thought of using a reducer to make a collector, but i have seen the boxed collector like that guy with the castrol WRC celica remake did.
the tube collector i would only reccomend if you got a really good saw,and make some solid jigs, or if you dont mind doing it over a few times.... my saw kept on flexing when i got close to the end where you're cutting the pipe on it's side.. mine doesnt fit up the best , so i'm going to attack the inside with a dremel later.
btw its sch40 316/316L Stainless steel pipe. most use 304, because 316 is supposed to be more expensive. but for some reason its cheaper on mcmastercarr, and 316/316L has better corrosion resistance at higher tempatures so it'll stay looking nice longer.