Hello! Sounds like you've got a pretty big project ahead of you. First question, why do they want you to remove title tags, descriptions, etc? I wouldn't remove them, I would do some keyword research and target local keywords specific to your geographic area. For example, 2016 Honda Civic Orange County.
What pages are showing up as duplicate? Are they specific vehicle pages? How are they duplicate? There are better options than rel canonical in many cases. Read this article and it should provide some insight. https://moz.com/blog/complete-guide-to-rel-canonical-how-to-and-why-not
Sometimes there is no way to avoid duplicate content, especially on a dealership site. Check out DriveTime's website, they do a good job on their SEO. If you filter for a specific make and model and look at the vehicle descriptions, you will find duplicate content. Compare two cars with the same make and model, but that are different colors and you will see what I mean. This type of duplicate content usually won't hurt your SEO, especially on a dealership site where your pages are constantly changing as you receive and sell vehicles.
You can always tell Google not to index specific portions of your page if you have duplicate content that provide no SEO benefit like disclaimers, warnings, etc. https://perishablepress.com/tell-google-to-not-index-certain-parts-of-your-page/
Also, check out this video by Google Webmasters. The speaker makes it pretty clear that you won't be penalized for duplicate content unless it is spammy. https://www.youtube.com/watch?v=cxWo4ttPgAc
If you can be a bit more specific with the duplicate content issues, I'll see what I can do to help.
Good luck!