It is quite unusual to have experienced so many occurrences of EML warnings on several different cars. Unless you are purchasing diesel cars that are very low mileage and being used for frequent short journeys and the EML warning are related to DPF issues, there is no obvious explanation as to why this has happened so frequently.

Choosing a higher mileage car is not necessarily a negative, as long as the vehicle condition is good and the service history is complete. Cars with average or above average mileage may have spent more time on motorway journeys, which are less demanding on a vehicle than frequent short journeys and stop start traffic, so you may wish to consider higher mileage vehicles in future.