Unless you have really specific concerns about the listing broker(s), I'd agree with Cam as to not really gaining anything by using another person.
Your SURVEYOR is working for YOU and should be able to ensure that the boat you choose is worth the pending deal. I realize this doesn't particularly help with the initial search, but these days it's easy to "see" a lot of boats yourself.
Sounds like you've done a lot of looking and you know what you want. So comparing your potential deals with other listings available on the various sites (remember there may be regional price variations) should give you peace of mind as you go ahead.
Remember that the info the dealer gives you was probably provided by the seller, who may or may not be overly forthright. Other than generic info on the model, your "buying broker" may not have any more facts.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)