Tide cold water clean works very well for cleaning really trashed car interiors. Just google "Interior detail with tide cold water clean" for videos on how to use it.
Colorx will do a pretty good job by hand if the paint isn't trashed. I think you have all you need right now. I'd love to see some before and after pics if you're able to take them.
I'm not sure about Massachusetts inspection since I live in PA. I'd make sure there's nothing major wrong with it mechanically if you decide to buy it. How many miles are on it? If it has very high miles that could explain the price.
As far as how that got there, I have no idea. They could...