any decent brand of polymeric calendered vinyl should do the job.
it should be applied dry and the area for graphics on the car should be properly cleaned and decreased prior to install. meths or isopropynol will do for that.
once applied, you should warm the surface gradually to promote a faster bond/adhesion.
other than that, pressure washers will be fine with it. however, you do NOT give any sort of warranty for letters blown off with a pressure washer. these things are used for cleaning stone, driveways, chewing gum off the ground and more. if someone is washing their car too close to vinyl letters it will break or come clean off. thats not your fault, thats the idiot with the washer. :awkward: