I have done a few of these and one I avoided due to its condition.
the ones I did do were in good condition and relatively clean.
cleaning is an obvious, but it’s not easy on these galvanised doors.
if you can give them a clean and feel confident they are clean enough to stick to. then fit your vinyl and slit every slat and fold in and out. you are not stretching or wrapping them into place as i do not think that will work. no need to anyway and quicker slitting.
you really need a heat gun so make sure you have an extension cable if the power plug isn’t close.
if the doors are painted and flakey, then make sure you point that out in your quote. that you are not responsible for flaking paint affecting the adhesion of the vinyl.
When i did my first one i took an old scrap of vinyl and tried a section just to see how slitting it would work and how i planned folding it after. no point doing it with a huge bit of printed vinyl and messing it up.
you have to remember that your looking at it from a distance and its not something your walking up to close.