I have never been to the any European PP dealer but I do know that H. Stern in America will not guarantee delivery of ANY watch to its dealer network. Therefore, they have informed their dealers not to accept any down payment as this may be considered a contractual agreement to supply the watch. I am not sure if this policy applies to other regional networks.
Depending upon the exclusivity of the watch you are considering it may be necessary to complete an application for a specific watch. These apply to minute repeaters, tourbillions and other limited edition watches. Essentially these watches are bespoken pieces. You may be lucky to find one such watch in a dealer but I understand an application still needs to be completed before the watch may be sold to anyone in particular.
The application process is a one sheet form which may be completed by the AD or the client. It asks such questions as the watches (and specifically Pateks) in your collection, your collecting philosophy etc. From previous interviews with Philippe Stern it seems he was looking for a steady progression of collecting from simple less expensive models to more advanced complicated pieces. If you are looking for a grail watch and do not own a Patek I suspect your application may be placed in the repository! I do know of a situation where someone was approved for a very exclusive watch and they were required to purchase a number of higher-end smaller complications to acquire the watch (this requirement was from PP and not the AD).
I suggest your best course of action may be to start with a few smaller watches and then move up to the big dogs. Consider purchasing a simple hand wind, than a steel watch, an annual calendar and than a chronograph. If you are looking for a watch with a price tag in the $300-$500K range it will cost you almost double that in the secondary market. This way you will only have to pay retail for the grail watch, you will acquire a nice collection of other Patek watches and you will be well connected for future watches. A solid investment strategy if you ask me.
Mike S.