You'll get most of the salt out pretty quickly.
I usually run it until the engine is at temp, then give it another two minutes then use the "blender" to siphon the Descaling flush through the engine for one minute.
Some guys have steel trailers here. It's no big deal so long as you rinse and wash it when you wash your boat (which is every time you use it)
Brakes never last in salt water no matter how good you flush them. i'd really recommend switching to disks if you plan to keep the trailer for more than a year.
The only bad thing about FL... You'll use that boat so much you'll wear it out, or you'll want a bigger one!