it will repeat the same thing TS, want lesser problem then buy new..
You buy an old car for sure wear and tear and replacement of part is needed.
Unless you buy a car like 3 years then you might able to enjoy problem free for first 2 year unless the previous owner did not take care well.
These drivc train and battery issue i believe not easy to diagnose.
leaking yes if its major, but as you said you used a few month only saw the leaking.

i know, but for new those wear and tear problem are more unlikely to happen.
320d is robust and its considered last one which offered here since no more diesel model after 320d.

it depend the "above national limit" all type of car will drop after 110kmph.
320d could go 900km full tank even tho you drive 160 for long distance.. driving this car we dont really talk about fuel efficient... even if you mention the fuel is similar with other car with petrol, but do those petrol car you compare have similar power of torque?