What aspect of the car are you trying to improve? Be aware that there is precious little you can do to get more power. Your best bet in a Benz is to buy the power level you desire up front. The tuners that can get you more power get $4k for minor improvemnts and $15-25k for real performance rebuilds. The Kleeman superchargers may be an exception, big power for less than a new engine.

Keep in mind that the V6 in the 320 already puts out in excess of 1hp per cubic inch. MB is very good at extracting all of the availible performance before selling you the car. This is rather contrary to the US brands.

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1995 G320
1984 280GE
1971 Unimog 416