£5000 would include the £2200 + vat bonus and dealer margin, he cannot physically offer any more on a standard car, if it has extras he could offer a few quid here and there.
As for searching dealer stock, yes and no.... They can search most stock however... people can easily "hide" cars from the system by marking them as sold or tagging them to ghost buyers. You will find if you ring around dealers you will find cars that wont show on the dealer network, this isnt just merc, its all manufacturers, we have to do it all the time when finding customer cars. Certain cars, dealers want to hide as they will have no problem selling them retail and making a killing whereas if it was on free release, it'd be snapped up by a dealer elsewhere in a matter of hours as you have effectively enlarged the cars audience to customers all over the country but regardless of whether there are cars out there, unless the car is spec'd up considerably you won't beat that 5k, its very good