Yes, 84 is a dual already. You are on your own on rust. I don't know where you are- here in Florida I wouldn't put up with it, but if you are in the rustbelt it might be different.

It's impossible for me to give you an accurate opinion with the few data points you have given me.

Presumably you have eyes and ears. Get the facts from the owners and the cars. Anything rubber on these cars must be replaced (or must have already been replaced). Timing chain attention and subframe are expensive, as are just about everything else on these cars. Look at some of the parts prices on this site to get an idea of things that can and will go wrong. Then try to figure out who is going to be able to fix this beast besides yourself.

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)