I'm of the opinion that the greenest car you can have is the car that you currently own!

No matter what you buy, energy was expended to make it! The car you own already has spent the energy. It's done.

So, assuming you're not spitting flames and smoke out wherever you go, your current car will be greener than quite literally anything you can buy, new or secondhand.

Note: I've done zero maths to verify this but it makes perfect sense to me :)