The minimum compression reading for the 240D is 17 bar or 255 psi. Another thing you might want to check is when its running (assuming it runs) pull off the oil breather tube at the cam cover and see how much air/oil is being pushed out. This is a good indicator of the amount of blowby that the motor has. I personally wouldnt purchase a car if more than one cylinder was at the 17 bar minimum reading because that would signal to me that the motor was on its way out.
Good luck with the car!
