I too prefer doozle.
Mpg depends on the vehicle condition, where you drive and how heavy your foot is.
Problems with the engine or vehicle in general?
Engine - at the sorta mileage you’re looking at typically, I’d prefer one that the vanos/chains have already been renewed.
Vehicle - should drive straight no wobbles at all speeds, no clunking on corners, shouldn’t feel bumps and no warning messages on dash ; particularly gearbox warnings.
Having a diagnostics plugged in would be ideal