The Mark 1 wasn't repurposed; Pepper and the Agents specifically find it intact in Stane's lab. And in the Iron Man 3 prequel comic, it was shown that Tony stripped the Mk II/VTRBS 001 back down to its original specifications for the display. So one of those is the oldest.
Now, there's some debate as to whether the Iron Man 3 version of the Mk I is supposed to be an original– Pepper mostly blew up Stane's lab with the arc reactor, so is the IM3 version a salvage or a replica?
But there's also debate as to how original the other suits are– for instance, the Mk III helmet was crushed, but the Iron Man 2/Iron Man 3 version of the Mk III displays it with the helmet intact. And its chestpiece is damaged, but all the other suits have their respective battle damage buffed out/replaced.
And just to play Devil's Advocate on the aesthetic side of things, the vast majority of the suits we've gotten in Lego are virtually identical red-and-gold numbers, so getting more gray would really still be changing things up.