+1 on the sticky post recommendation.
Personally I wasn't searching for a pristine example - price was much more of a concern than a couple years of salty use for me. I might have felt differently if I was looking at a 15 year old 100-series though.
What's the recommended rust protection
for those of us in the north? I've seen mixed recommendations, from "weekly undercarriage wash" to "paint everything underneath" to "spray with motor oil (or an equivalent oil/wax solution) 2-3 times each winter". I don't have a garage, so whatever I do will need to be done at a reputable shop.
Fluid film application. You can have shops do it or do it yourself. Do it once a year. Thick coating of non toxic lanolin based fluid.
You can buy it in bulk and use a sprayer or it comes if aerosol cans if you want to do it yourself.
I spray all my trucks and live in an area with heavy salt use and it makes a huge difference.