OK, yes, I'm afraid in this respect this example may not accurately reflect my real problem. One of the reasons I wanted the class "VehicleDealer" to have those attributes is that it should be able to verify that it has all the information it needs to forward the real work to the Builder. This is because the actual job may be time consuming and I want to be able to throw errors immediately if the parameter set is incomplete or inconsistent, but maybe that's not such a good idea after all...
I guess I could easily separate instantiating and running the builders so that I can have them check their parameters before running the job. Thanks for your help!