The comments highlight the two extremes of how Post is done.
On the one side there is the -what I call- “damage control”.
The Audio Post Studio “fills” up anything that is missing from the location sound, in order to obtain a consistent audio “stream” throughout the movie/series.
In this case the client should be very aware of the limitations of this method. Delivering an M&E for dubbing in another country is simply out of the question. (Actually, we are building a good business on building M&E tracks for Tv series which are sold to a foreign country.) For Sunshy this means leaving the DX track untouched, and just adding what’s missing. Of course that limits the mix- and many more compromises need to be made.
On the other hand, there is the “High Quality” Movie-way_of_doing_things. Which means that everything is “re-done”. So, as soon as you mute the dialog track, the whole M&E track completely stands up and is complete/ready for foreign language dubbing. This involves splitting up the location sound into a PFX, BG and Dialog track, and then later “fill” anything that is missing. For Sunshy this means, replacing the car and performing ADR on the dialog. This procedure also gives more mix options.
And there is everything inbetween. As for how to approach it, whatever works best for you. But you need to be sure that you are in agreement with the client, that he is aware of the consequences and limitations. Otherwise you will be in big trouble. As for us, even for “damage control” series, we put in many extra work for splitting everything up into different stems, even though some parts will be missing. The advantage is that it results in a much cleaner project, easy to mix, easy to find stuff, and whenever an M&E needs to be build at a later time, you are already halfway. That is our personal “investment” in each project.