The long story of each car type lives in Sanity, and Mellow keeps its facts: specs, pictures, features and the price. An editor picks the car type in a string field whose input reads Mellow's catalog feed. The page then shows the editor's text beside the widget for that car type.
Not tried yet
Mellow has not tried this recipe in a Sanity Studio yet. The input follows Sanity's documented API for a custom input component.
The catalog feed
In Mellow, open Settings · Widget and the widget. Under Show more, press Copy CMS link. It is the address of the widget's catalog feed, such as:
Read carType with the page, such as with GROQ *[_type == "carPage" && slug.current == $slug][0]{ title, story, carType }, and give it to <mellow-car-type id="…"> and to the search's car-type, as in One car type on its own page. The Prismic recipe shows that markup in a React component, and the Nuxt recipe the same in Vue.
The code is the join between the two: staff may change a car type's code in Mellow. Then the editor picks the car type again. To join on the id instead, which never changes, put c.id in the option's value. <mellow-car-type> takes either.