Welcome to X5 World!
Here's my take on your questions -
At the present, the BMW dealers have a ton of remaining '05 X3's and X5's to move. BMW will likely gear their price incentive programs towards leases instead of retail purchases. Since the '05's are virtually the same as the '06's, I don't see BMW as being in a great rush to move them. Next year might be a different story when the '07's come out.
Executive demos are driven by BMW employees up to some mileage limit - I believe it's somewhere under 15K. Beyond that, the car can no longer be sold as a new vehicle. For the most part, they are probably good condition vehicles, but there are probably exceptions. Have them pull the service history, check to see if there any major repairs done while in fleet use. Ask for a CPO type inspection to get the status of all major parts and systems. Do the same with CPO vehicles - get a copy of the full CPO service record and service histories which the BMW sales people can pull up immediately from their internal website.
My guess is that the Gen 2 '07 X5 might impact the '00-'03 vehicles, but won't impact the '04-'06 facelift models very much. The '04-'06 facelift models already have the latest and greatest N62 Valvetronic engines, 6 speed automatics, and X-Drive. Unless the '07 X5's have some new great, spectacular features with a "gotta have it" factor, I don't see how the '04-'06 facelift models are going to fall down greatly in value. It looks to me like the new X5's may end up looking like a big X3. Throw I-Drive into the equation, it make take awile before the new X5's gain acceptance.
Don't be afraid to shop in Southern California - there's plenty of BMW dealers in a very small radius. What better way of enjoying a new car than by driving up home up Highway 1?
Regards, Mike