Morning Roy
What are you trying to achieve?? Better Ride ? More Grip ? Quieter Ride ?
Runflats are well known for delivering a harsh ride, particularly towards the end of their useful life, as well as more road noise.
So if you are keeping the same wheels buy the same size tyres but non run flat, just don't buy cheap. Tyre footprint is about the size of a saucer, and that's all there is to accelerate, stop and steer your car. I always stay with recognised brands I am on Yoko's at the moment 275 x 40 x 20 Front 315 x 35 x 20 Rear. My brother inlaw has 535d touring and runs 245 x 40 x 19 Front 275 x 35 x 19 Rear Bridgestone non runflats.
If changing wheels to try and improve ride quality by fitting tyres with a taller side wall Brake size will dictate what will fit (18" I believe) so something like 245 x 45 x 18 may give a more forgiving ride
Dave