"Ideally, we wouldn't want to exclude these pages from being indexed because they could have some long-tail search value. But, obviously, we don't want to hurt the overall SEO of the site."
You say that, but I'm not entirely sure it's true.
I understand the theory - if you have 20 Citroen C1s listed on the site, you could potentially have 20 pages of yours ranking for relevant terms, right?
Well, unique content on those pages or not, I think it would be extremely unlikely that Google would want to present all of those results to the user. Furthermore, if the pages expire or go "out of stock", as it were, when purchased, would Google want to rank it?
So I'm not convinced having all those pages indexed and treated as unique (whether they are or not) would result in traffic (please prove me wrong though - if you have lots of entrances to the site via organic search to those pages it'll show what I know!).
My preference, regardless of the above, would be to have a main page for your Citroen C1 products - a hub page - that then links to all the different products you have as and when they're available.
This has many advantages - you just need to focus on ranking one page in the category instead of several, you can collect all the link equity you earn to one page, you can ensure the page is well optimised for search engines and users, and the page will be evergreen - meaning your links would be too.
The short version:
Homepage > Hub Page > Product variant 1, variant 2 etc
Rank the homepage and the hub page.
Hope this helps.