dont get them powder coated, get them jet coated. it's a different type of process. look into it if you can.
to answer your questions, if you want to do everything in 1 inch, you'll really need 100 ft. if you only want to do each header pipe seperately, 50 ft for the 1 inch will be enough (note, be conservative on each pipe to ensure you have enough cuz you'll use all of it). and then you'll only need 15 ft of the 2 inch to finish the rest of the headers where the pipes join together.