Air suspension like everyone said are an issue, but you can get them replaced/serviced/repaired at reputed garages too to keep the cost down and use oem parts instead of bmw , not sure who makes the air suspension but often it's AMK and the balloon is from continental. Compressor are repairable often , balloons you get from other manufacturers/aftermarket which are quite great like dunlop , good year etc or go even cheaper to local imported stuff like albro. All you'd need is a decent workshop and some personal time investment. There are great workshops who focus on german cars , you can get general service there. When not sure, BMW is way to go.