For the underbody... you can sand the rust off and paint it, this is what I do. For some parts, you can send, paint, and/or plastidip them. I may actually try this for some things.
Also, what if you have a very subtle whine in steering (in one direction) that seems to go away after it warms up? I noticed this after my last trip to the Uwharrie. There was no steering fluid, so I topped it off.