All DI motors, N55 included, will have carbon build up on the intake ports over time. Figure a $400-600 cleaning every 30-60k.
The older (N54) motors had issues with the fuel system, turbos leaking, and were generally more prone to issues and BMW extended the warranty on some parts. The N55's seem more reliable. All newer BMW's have electric water pumps that will shit the bed somewhere around 60-80k, budget $1000 to replace. My 135 has been trouble free, but I've only put 8k miles on it.
If maintenance and cost of ownership is an issue, you may be barking up the wrong tree here unless you have the budget for a CPO car.
The older (N54) motors had issues with the fuel system, turbos leaking, and were generally more prone to issues and BMW extended the warranty on some parts. The N55's seem more reliable. All newer BMW's have electric water pumps that will shit the bed somewhere around 60-80k, budget $1000 to replace. My 135 has been trouble free, but I've only put 8k miles on it.
If maintenance and cost of ownership is an issue, you may be barking up the wrong tree here unless you have the budget for a CPO car.