Cars sold not up to standard

Hi All, I bought a car from them 3 months ago and I'm having endless problems with the car cutting out. I bought a BMW 320d e90 2008 model from them and they are supposed to be specialists in BMW trading. This is my 4th second hand car that I have bought and not once have I had to struggle with a car like this. I know what I'm getting myself into with a second hand car, but when it is serviced adequately and to the correct standard then you don't expect electrical and mechanical issues to the extent that I have experienced. I was convinced that the car was serviced privately after the warranty period but when you find water in a radiator system, in a car that comes out standard with coolant/antifreeze then I start to question the validity of the information I was given. The car has been standing by a mechanic for the past 3 weeks and does not even want to start, all of this after only 3 months.... I'm also planning on taking this case to the ombudsman - City of Cape Town as this cannot be accepted.