Would be actually fairly easy to do. Of course depending on where you customers might be coming from. If it is national (US) you can get a ZIP database from various locations over the net.
I created a
distance calculator some time ago which I never cared to make too accurate but could be done by refining the formula.
Basically the only difference to your solution would be that it searches all dealers in the database against the origin zip, creates a set of all dealers that are within a specified range (or just the closest) and use any email object to email that information.
Let me know if you want above script to get an idea about the zip code calculation (us only).