dont get a exhaust trim mate.it would point down anyway so a back box is your best bet.can pick up stainless steel ones pretty cheap especially universal fit ones.
as for the bumper being modified for the back box,depending what sort of back box you decide to get you may need the bumper to be modified which is not a big job it is literally cut out lol
the paint job would vary in cost but it would proberly be around or over the 1k mark.
hope that helps a little bit matey
welcome to sax-p by the way!