Gino is right, paint is a good option too. The only thing that would suck is the prep time. good scuffing of the surface (we're talking, 3ft x 16ft for 2 sides, plus probably 3x5ft for the back, right?) Will take some time to get that scuffed. Anyone that's painted anything knows, you don't scuff it good, that stuff will be ripped off after applying tape to it.
Im looking at it this way. From how you describe this 16ft trailer, it's a POS, the guy wants to get 5yrs max out of it til he throws it in the scrap yard. So, already, it probably doesn't look the greatest. You could put cast on it, sure. The vinyl will outlast the trailer. 651, will outlast that trailer. Cut where it intersects on the metal, do a good appication, be done with it. I wouldn't waste a lot of time on this job, and just get it done. It's probably 4ft wide sections of metal, giving you 4 panels, so cut a few 52" and cut em off inbetween panels to make it easier to deal with. Slap some letters on that and kick it out the door.
I know calandered vinyl isn't the best for vehicles, but people look... don't compare wrapping a ford explorer or a dodge ram 1500 to a flat, no rivet trailer. There is no possible stretching here, no dips or bows in the bodyline to worry about. If you're affraid to letter a flat surface with 651, are you telling me you letter flat aluminum signs with 751 cast vinyl? It's flat, easy as it gets. Don't get crazy, just get it done. 651 or paint. I'd do 651 though. scuffing up a total area of around 37ft by 3ft, even with a 600grit orbital is going to take a little bit of time. Plus dry time. Or, just apply about 13yards of 3ft vinyl.