Dealer just finished replacing water pump, thermostat, and coolant expansion tank. While they were servicing, they also recommend replacing mechatronics sleeve and transmission pan gasket due to transmission fluid leak. I have search the forum and it seems that this is a common problem with e90 auto. I haven't notice any fluid leaks when parked overnight. Want to know how urgent should I have this done and what's a reasonable cost to do it (dealer or independent). Is this a DIY that is difficult and easy to screw things up big time? Any input is greatly appreciated. BTW, my car is a 08 335xi with 117k miles. TIA.