I'm planning to buy a Merc diesel. Which diesel is the best overall in regard to price and availability of parts, price of repairs, gas mileage, inherent weaknesses, and longevity/dependabililty? I only have up to $4000 to spend, but have seen quite a few good to excellent '60s-mid-80s Mercedes diesels on Ebay and Auto Trader in that price range. I'd like as much feedback as possible, so don't be shy.