The best test of the condition is to start it stone cold in the coldest weather available to you, like first thing in the morning. It might stumble a bit which could be just a bad plug, but if it fires right off take that as a sign the engine has many more miles left in it. If it really is an OEM trans for that price, I'd jump on it. About that mileage is when ppl start to have trans issues so if yours is new you should be good to go as far as really major repairs for a long while. As long as the t-chain doesn't have excessive stretch, it doesn't matter if its original. Excessive stretch, however, is a matter of opinion.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo