Hi folks. We were creeping Google and stumbled onto this post. We decided to make an account to offer an angle.
First of all, you are trying to bend basic page builder into an inventory and/or object management system. Let's face it, wordpress is a public comment system not a management system.
Second of all, forget all the selling points of wordpress and realize its feature set and scalability. Any frame is seo friendly. Any frame has easy menus with wysiwyg editors. This is year 2013 - the entire feature set of wordpress has been successfully mimic'd well over 50 times.
Third, if you're gonna do this open, use drupal. Think feeds... Get your InvRM to export xml, csv, or even Google base txt. Use views or bend drupals news feed system to craft display nodes from your feed files. Make a secret ftp for drupal and/or your web servers wget + cron schedule to pull updates. Set subscription alerts so when content changes you can easily monitor it via a salted catch all email.
You can also bend good open commerce solutions to do this for you. Just make "products" the cars... Hide pricing, payments, and cart stuff.... Nopcommerce was a good one we benchmarked lately.
There are other ways but like yourselves, we are all bout diy. Just thoughts friends! Good selling!