As a long time user of cast metallic films, I would suggest to go with 3M Scotchcal film or Oracal's premium cast for the film itself. To extend longevity, the very best stuff I use for my big rig clients is Premium Urethane Automotive Clearcoat. I've done tons of airbrushed graphics for vehicles since '95 and those still on the road are in very decent condition. Sure I did this by the time I was in the spray booth but you probably have a bodyshop in your neighborhood so maybe you can ask 'em to lay down a coat on your vinyl. Usually spray booth are about 20ft long so you can prepare few sheets of 24" x ???" knowing what lenght you need to cut your nested letters / graphics.
Just a regular coat is more than enough. You will easily double the durability
* before you ask, any brand can do the job as long as it's premium stuff (DuPont, PPG, Sherwin-William, Sikkens...)
** no special preparation needed. I always just wipe down vinyl with alcohol or PrepSol and spray but if you want to spend a li'l elbow grease, use 3M grey pad and wipe the dust off carefully.