i have 320 2009 coupe. so far so good. mileage now 65k km. only changed brake pads. not aware of any major service due soon...in fact none as per the computer print out.
fuel consumption....i wouldn't worry. if you can afford to buy a premium car, you shouldn't be worry about fuel consumption. just enjoy the ride.
Power wise, 2l engine is less power (not underpower) than 2.5l. 2.5l less power than 3l. 3l less power than 4l and so on. My point is you get what you paid for.
one consideration, if you have to sell the car later, the recond one will have slight problem to sell as compared to local import. people will ask so many questions regarding mileage la bla bla.