I went through the same battle with my Excursion, It was a 2000 limited 4x4 7.3 diesel. It had 306,000 miles on it when I sold it. The interior was immaculate and mechanically perfect. New oil pan, tires, injectors etc.. The only issue like you said was it was never undercoated or had any type of rust proofing done. My door bottoms where gone and the rockers had large holes starting as well as cross member and floor pan rust. For my taste is was too far gone to spend more money on. I wound up selling it because with the heavy salt use up here It would need major attention in the spring.