Suggest script for a new website

Well, to begin with you are going to have to have a database, that's number one. There are a few scripts like boonex dolphin for membership sites in which you can limit memberships to certain groups and you could build the site down to your needs (overkill but effective) or you could try joomla and drupal as well with a few modules (solid platforms but designers can be expensive). If you are looking for a simple and decent way to do this and get out of it on the cheap, I would suggest that you build a wp site and add buddypress to it with s2 member or wp-e member installed behind it. If you wanted to go a step further (Profit Margin), just install mu with it too and provide some added abilities for those car dealer members. I have 2 or 3 wp, buddypress, mu sites running now and I can tell you that you can pretty much get away with anything or any configuration you want with a little pre planning and determination, lol.