Advice on how to buy a presidential

[Onyx]

So I like the 36mm style yellow gold president and like 2001 model year or similar style. I talked to a local dealer offering a 2006year d serial factory yellow gold black onyx diamond dial for 20k with shipping. How do I know if that's a good deal? what would you guys do if you were looking for this style? I'd like to pay less than that 20k watch. It had original papers too, no box.