I've run a 54 plate 120d for 30k miles and it's been a fantastic little car.
First things first, if the previous owners haven't already, ditch the runflats. They make the ride absolutely awful, and don't do much for the handling either.
But other than that, don't get a sport, go for at least the SE or the M Sport. SE comes pretty well specced, but the M Sport looks a lot better (although to me I prefer the look of the SE, the cardboard insert looking rear bumper looks awful on the M Sport). The handling on the SE with the softer suspension is still brilliant and can provide great fun throwing it around some B roads. It also means with the state of roads, going over a pothole doesn't put you in A&E
Options to go for, iDrive if you can although it's rare, especially at that age and budget. But Professional Headunit at the very least (I think thats what it's called), the standard audio is turd, but even the first level upgrade on the audio makes it worth while. And at 56, you'll get a sub under each seat (after they facelifted they only had a sub under the drivers seat). Climate control, multi-function steering wheel are other musts, just for the look of the interior and obviously the functionality they add. Couldn't live with a car without climate control now
120d returns decent fuel economy, I'd see 48mpg on a few trips up to Darlington and back. Also my commute to work would return 45-48 for the most part. Town driving and cold winters you'll see low to mid 30's which kind of sucks.
Only issue I had with mine was rattles, the interior is full of them especially in hot weather. I've had to put cable ties in between certain panels to stop me from burning it to the ground
Reliability wise, in 3 years I've not had a single bill. Although most sites suggest they about average in terms of reliability