White paper justifying the publication of a survey paper
If I understand your question correctly, the longer survey paper is mostly or entirely written already, right?

The purpose of a presubmission like this is generally to help the editor decide if the topic is broad enough and well enough aligned to be of interest to a large fraction of the journal's audience.

As such, the introductory material that you've written for your survey paper should be just what is needed: it should already give the scope of the survey, the motivation for it, and an outline of how the rest of the survey is structured.
I would recommend clipping out your abstract and introduction, keeping its references, maybe adding a few other key references if your introduction was reference light, and sending that in.

Finally, in your cover letter, you should say that this is exactly what you are doing.  That will also let the editor know that this isn't a submission in advance of having written the manuscript, but an inquiry of whether to proceed with a fairly mature manuscript.  Speaking as a sometime editor myself, that's very useful information.