This is an interesting comment from Yahoo answers where I posed a similar question..
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After owning the 200 tdi Discovery for a number of years I must disagree with the answer that states Discoveries don't rust because they do. The outer body panels are aluminum and suffer from aluminum corrosion, the chassis and outriggers are steel and suffer from extensive rusting, especially if used off road and within the Farming community (cow muck and uric acid will rot most metals), front and rear bumpers are also steel and corrode after a few years and most of the inner panels and hidden frame work is steel and also prone to extensive corrosion. Main areas for rust are as follows :- Rear inner wheel arches, Rear luggage area floor plate above fuel tank and the main chassis side members above the rear axle. Fuel consumption on the 200 and 300 TDI's with the manual box averages 25mpg the auto box gives slightly less.
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Am I really looking at a rust bucket if I buy a 97/98 300TDi ?
Pete