What tool should I use to cut metal roofing?
I used to use a 7 1/4" metal cutting wheel on a circular saw. Works good, but wheel wears down fairly fast.  There are also some good shear attachments that mount on drills.  I'd advise you to make the cuts on the end going to the top, not the exposed bottom. Definitely have some spray paint or primer to treat the cut end, because it will rust. If you put the cut ends at the top under your ridge cap, you will be fine.  The other alternative is to buy your panels cut to length before they land on the job. Several manufactures cut to order such as Everlast Metal Roofing.