1. Will the price be much different if I buy it now, or say 2012 Feb?
Use Kelly Blue Book to determine depreciation
Official Kelley Blue Book Values You Can Trust From Kbb.com, The Trusted Resource - Kelley Blue Book
Also, if you're asking this question that means you can wait. If you can wait, why not wait and save some $?
3. It will be from Elliot Auto broker, have anyone got any exp? If not, what should I watch out for?
No experience, but you should always carfax the car.
4. 2008 535i for 25000USD, or 2004 530i for 15000USD with half the mileage 30,000. Which is better value?
Assuming that value means the fair value after you use it, you should go with the 2004 model. Since the 2004 model has already depreciated so much, after 4-6 years you end up with less depreciation than the 2008 model. Also, 30k milage can mean less repairs for you, in comparison to the 60k milage on the 2008.