We've used LinkCollections for this in the past, but this is a really flawed method, because they can pick ANY page (not just related car models), or they could add links to random hyperlinks, which won't help you either.

EPiServer has the Page property, but that only allows one link, and it doesn't allow you to limit what they link to.

To do this, we generally write a custom property.  This allows us to:

1. Limit the PageType they can pick
2. Limit the locations they can pick from
3. Allow them to pick multiple other pages

EPiServer really needs a built-in property like this.  You should be able to have a "Multiple Page" property, from which you can limit selections as I noted above.

Intergen released a Multi-page Property, but I didn't see how that was much different than the LinkCollection (very good chance I was using it wrong).

Itera released a Multi-Property, which allows you to roll-up multiple EPiServer properties in a single "container" property, which would likely work for you, but it still doesn't solve the problems of limiting selections.

Like you say, a custom property is the way to go with this one. I'd consider storing the car brands and models as categories, and using the properties settings for choosing a root category and listing its children in the drop down control (see http://www.frederikvig.com/2010/05/episerver-custom-property-with-custom-settings/). You then need to write a little JavaScript code that listens to the select elements change event and populates the model drop down (you can also use the OnSelectedIndexChange event on the server for this).

Hope this helps.

Frederik