Power wash the underside as best as possible, get a good rust encapsulating paint and go nuts. Paint the entire underbody if you can, it won't stop the rust completely, but it would slow it down a lot.
we have a 2010 FFH with 135,000 miles on it now, got it used for $6M in 2017 with around 85M miles and have only done tires, brakes front and back (although the fronts did not need to be replaced) and the B1S1 O2 sensor and throttle body. Oil changes around every 3500-4000 miles with full synthetic, I have run flats on it since this is pretty rural here and I don't want the Mrs. to worry about tires. It is the best running, most reliable vehicle i have owned in 60 years of car ownership, and still gets 36mph or better consistently even though it is now a 14 year old car. I just did a rebalance
(autel md808) on the HV battery yesterday, and all 28 groups are within .04 volts of each other when completed, showing again how good these battery packs are even after years of extreme heat and cold. I know it won't last forever, but these are really great cars imo.
