The rust won't stop unless you get rid of it. That means either cutting it out and replacing it with good metal, or find new or used replacement doors and simply swap them. The rockers are (for me anyhow) easier to replace that to just patch.
The spray foam idea is a BAD idea. Again, the rust will not stop unless it is gone. Plus at some point it will need redone. And if you ever decide to sell the truck, this type of repair will damage the value. FWIW.... back in the mid 80's I had a 50-something Dodge delivery that we took to a "body shop"
to fix the rust above the back wheel wells. He never cut the rust out, shot if full of expanding foam, shaved it down, bondo'd over it, and primed it. When we got it home and backed it off the trailer, as soon as the rear tires hit the ground the whole bondo/foam craperoonie fell out.
Those seats are not a bolt-in deal, you will need to have custom brackets made for them to fit. Not hard to do, but just know they won't just plop right in.
Lowering on the cheap? Cut coils, spindles and moving the axle to the top of the springs works well. Look through the classifieds here, you can probably pick up springs, spindles, flip-kits, whatever you want for a decent price.