The 3 Series Coupe works very well because it's not only good
looking, it holds four comfortably and takes four people's luggage. The A5 and E Class coupe are not quite as capable. Rear seat passengers sit uncomfortably more upright and there is less headroom. 20k miles a year probably does justify diesel. The current 330d is reputedly as powerful as anyone needs, and very economical as well. Make sure you go for the smallest wheel size with deepest profile tyres. Definitely do not go to 19" wheels.