The above book is excellent about the earlier guns - do read it. He is also a dealer and has a website. I'm not clear if you were after a new gun or a secondhand gun. If new then your price level will be tricky. There are some makers now that use Italian or other parts and then finish them in England, but that may not be considered a "best gun".

I went round Westley Richards in the summer, and would definitely buy one of their guns if in the market, but I suspect it would be a lot more than $10,000.