My current daily driver is a 91 300d diesel with 277,000 miles. I do 30,000 miles a year. It is what they call the 124 chassis car. It was a one owner and bought it with 230,000 on it, with all the service records. Truly....the little old lady's Mercedes.
Personally, the 123 chassis (up to 1985) are too old to drive a lot. They are starting to creep up in price each year because of the collectible status, particularly with low miles. They MIGHT do the daily errands just fine, but repairs are not cheap unless you find a garage with parts cars on site.
I'd pick a 124 chassis (1986 and up) and get one with as few miles on it as you can afford.....the lower the miles the higher the price. I have in my sights a 1995 300d with 70k, but the guy wants 10k for it.....a bit too much. I don't mind paying a premium for condition and low mileage but that is too much.
Get one with the service records so you know it was taken care of. My 1985 300d was sold with 414k on it and I had all the service records since I bought it with 125k on it.
I have been driving MB diesels for 35 years an love them.
There is something very Zen-like from the hum of a diesel engine at 75 mph.