Can't say I'd want to drop £20k on that tbh (I'm 43!) my money would be a late e39 M5 and £10k to run it!!
Clearly though, an M or indeed something "special" doesn't appeal to the OP or his driving style I'd imagine, a warranty backed low mileage late model 3 is quite a car for the average 24 year old by a long way (even if it's a 318d), presuming he can afford it, why not?
Not everybody gives a toss about engine size etc seems to me the OP wants a nice car, not bothered about speed etc and does the mileage to warrant a diesel....
Again, not my cup of tea, but good luck to him if it floats his boat!
You pretty much hit the nail on the head, fast car's don't really appeal to me you can't drive fast these days anyway unless you like driving on NSL country lanes or live in germany with the autobahn. As i say i currently have a 120d m sport 2011, i find that it has more than enough power for overtaking and easily keeps up with traffic. Its only when you get to speeds well above legal limits that "performance" car's have any kind of significant advantage. Ive only picked out this 318d because its local to me and its well under 20k more like 17k.
The way i see it is its not 17k down the pan i won't be keeping it any longer than it has warranty, sure i will loose some money with depreciation but to me its not huge price to pay to be in the latest model. Personally i just like to be a bit different, most of my friends and most other people my age all drive sporty hatchbacks which i have also but i kind of like the slightly more upmarket laid back cruiser.