first i would fix the body damage and rust,covering it up does no good .the rust spots will continue to get larger and body damage makes it easier for rust to develop.I know how the northern states are with the salt in winter(i live in new jersey), and when the salt gets behind the armor it will make the rust way worse.
To fix it remove all the rust and if you have a welder or acces to one weld a small piece of sheetmetal behind the hole. Then fill in with some type of filler(preferably fiberglass). let it set up and then sand it until it is smooth and even with the surronding contours of the body.While you are at it remove any dents that will be covered by the armor if you can,if not sand off all the paint and fill them in also.What you want to do next is sand all the paint that will be under the armor to dull it,that gives the new paint (which i will go into shortly)something to bond to.Primer all the filled in spots with several coats(spray bomb will be fine,)when it dries sand it smooth.Now comes the paint,to prevent any rust from starting again use a high quality rust preventing,chemicle resistant paint,hammerite comes to mind.Hammerite is an industrial paint that when dry looks like it has been beaten with a ballpeen hammer.It is chip proof,chemicle resistant,and will not allow any rust to occure(i know from experiance )It is very simple to apply no need for a paint gun you can use a roller,in fact that gives it the beaten look.
I have no idea what gauge the pre fab is,but your 4 inch cutting wheel would be fine.just have extra wheels on hand,a face guard,safteyglasses,and gloves to protect yourself would be wise also.to fasten it on i heard of people using poprivits,sheet meatalscrews and for those with alot of time drilling holes and running bolts through.
if you are not comfortable with bodywork go to a scrapyard and pick up some sheetmetal pieces. they dont have to be very big just roughly 12"x12" or so,large enough to practice filling in dents,or patching holes with a welder.
Good luck on your prodject and keep us informed on how it goes,with pics if possible.
