Matt, my LS400 sounds like it's in similar condition and over the past year I've spent a fair bit on suspension work, servicing and under body protection. It was worth it because the under body was very, very clean. Most of the items you identify are par for the course for any older car. I need some paint done but it doesn't bother me unduly as the car is so damn heavenly to drive. I've formed a bond with this car, as has my wife, and several splendid driving holidays means I am saving for the paint work. It doesn't all have to come at once.
As for longevity, I expect in 10 years time these old barges will only be able to be run on a classic dispensation of some sort and won't be viable daily drivers. Only the best examples will be kept by enthusiasts. If you want a top notch one, import a Celsior that hasn't seen any salted roads.