1999 bmw 523 2.5 E39 model?

hello my neighbour has bought a bmw E39 model with auto transmission ,i did the brakes all round replacement for her , but she said that the gears are having problems changing up ,it starts of in 3rd gear instead of first as their is a yellow lite on dashboard illuminate which is showing that there is sumting wrong with gearbox, and also the A.B.S. lite is on, no reverse lites come on either, fuel dial does not work ,nor does the speed and rpm clocks. ,i put code reader on car and shows upPO700/ transmission control system malfunction., not sure if car needs new transmission or ecu module,,, the bmw drives fine except for the fault code and tranny...not sure if any-one had similar problems or what could i be looking out for,,, should i change the tranny oil filter and oil.. ,is there a sensor in tranny that needs replacing or new g/box... thanks for any advise, much apreciated.