You probably want to create a sitemap for your site. ILSA can provide you with the information needed to get the URLs of dynamic pages backed by ILSA.
You can use the /dropdowncontents call to get a list of all your makes or models and generate sitemap entries based on those.
You can use /searchresults to get a list of all your vehicles. Normally /searchresults is limited to 100 results per page, but if you create a fieldset with type SITEMAP in the Backoffice that limit is lifted.
You can use the fields structured_data.sitemap.url and structured_data.sitemap.lastmod for <loc> and <lastmod>.
Sitemap fieldsets are restricted to only a few fields to prevent abuse.
TIP
It is generally advised to keep a single sitemap limited to 50.000 URLs (and 50MB). If you might go over that limit, you should split your sitemap and refer to each chunk with a Sitemap Index. Read more at https://www.sitemaps.org/protocol.html.