as the question asked was "how good is a 124 diesel". then the answer is very reliable and very well built. it is slow though! The engines are the "bomb proof" variety and will run forever if maintained well. Also has the benefit of running on "comedy fuel" if thats your vice
I'd opt for the 3 litre version. there are plenty out there.
i have a 1989 e300d w124, uncle owned it from new and hass 700k+ miles on origional engine an autobox and is slow but not that slow if u know what i mean