I'm not aware of any endemic problems with the OM601, its proven to be a solid engine. Its also a total departure form the earlier OM61X engines so if you have tools and experience with the older engines not much of the technology transfers.

The W201 chassis is quite a bit smaller that than W123 you're driving, its been my experience that taller people don't fit too well into the W201 especially in the back seat.

I had an 86 190D that I used as a daily driver for 6 years and I was very happy, I routinely would get 40+ mpg using conservative driving techniques.

The first couple of years of the W201 chassis had some strange features that disappeared in later models. IIRC the first couple of years had 14 inch wheels, a small wiper located under the main single wiper blade, a lower dash panel that floated above the center hump and an electromagnetic fan clutch instead of the more common visco-clutch setup.

I agree with Barry - when you are buying a car that is getting on 30 years old condition is everything. I'd look very carefully before I'd drop 3K on an 84 190D.

__________________
98 Dodge-Cummins pickup (137K)
13 GLK250 (157k)
06 E320CDI (341K)
16 C300 (89K)
82 300GD Gelaendewagen (54K)