Yes, the process is pretty similar to buying a car, like you described.
I do have some experience with this particular AD and I have to say - much depends on how desperately they need your business. Allow me to share my story:
As a visitor from Hong Kong, I wandered into the Patek boutique at Old Bond Street some four years ago (I was a relative newbie in watches then) and asked if I could get a 5712G. None was available at that time. The salesman I spoke to offered me a 5101P instead, as they had one in stock. Not willing to spend that kind of money, I stupidly declined (I am still kicking myself today). Eventually, I opted for a relatively inexpensive 5119R and the salesman promised to put me on the waiting list for the 5712G, but indicated that it would take at least two years for me to get around to it.
I did not pin much hope on the promise, especially when the salesman subsequently left the boutique as well.
Approximately two years elapsed and one day, I miraculously received a phone call from a lady in London asking if I still wanted my 5712G! I was pleasantly surprised but unfortunately I already got mine so again, I gratefully declined. In other words, the ordering process does work! Well...to some extent...and no, you do not need to make a downpayment.
My guess is - if you wish to order something like a 5102, 5950 or, for that matter 5013, 5074 or 5078, you won't be entertained but if you set your eyes on more 'ordinary' pieces like 5980, 5205 or 5170, EVENTUALLY, you will get your turn, assuming you don't have an existing 'relationship'.
Let's hear from the experience of others.
Cheers
Sam

are 110% friendly, and wonderful people.
they are also extremely knowledgable, and are certainly not naive. they know their business well, I can assure of that.
They will assist you in anyway possible. they will not prejudge in anyway.
I know this from personal experience.
For them customer is king...
however, remember what I said, they know their business better than anyone...
so please dont walk in and say you want a 5131 or a 5170....and expect , they have a list as long as the M1 for this.
but if its an annual calender, you'll be in luck.
Best Imran
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.