I also have that same diecast and I also have the dirty version. Both have never been out of the box. I think I paid 150 for both from another collector.
great detail on the Coronet. do have any pics of how you modify any of the models? I was showing my grandson your pics and have him interested in building them other than stock.
He thought the 300 was really cool.
I just ebayed valiant die cast and a few came up.. Don't know what scale you were looking for
http://www.ebay.com.au/bhp/valiant-model-car
They are all from Australia.