You didn't say which TD you would like. There is the original W123 chassis TD ('78-'80), TD Turbo Diesel ('81-'85). It has a 3.0l 5 cyl. engine. Then there was the 300TD Turbo (1987), it had the 3.0 6 cyl turbo diesel engine. For starters, it isn't to find someone willing to sell their TDs. Just look in your local paper and see how many you find. The W124 chassis TD is even harder to find, since they only brought about 1100 of them here in 1987. The W124 TD really hauls compared to the W123. the engine is rated at something like 143 hp and 192 ft lbs of torque. (I think). I was very fortunate to literally stumble upon my '87 TD.
Alot of it is depending on what you are looking for. The W123 has the "classic" Mercedes-Benz look, with the curved fenders, bundt cakes rims and chrome.
The W124 has a "modern" look despite the fact the design is now 17 years old.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member