The schedule is listed in the service books, and show's what is carried out during each service. A specialist can do it, there is no hard and fast rule over that unless you have a BMW warranty in force. The inspections and services are called up depending on how the car is driven, so drive it carefully, and the intervals increase, drive it hard, they decrease etc etc, Mine follows the following (up until now): oil service, Insp I, oil service, Insp II etc, plus the annual service when it needs them for brake fluid and coolant changes. (at BMW dealers, during the Insp II, the valve clearances are done, where as at a specialist this is sometimes skipped as it is said to be unnecessary? don't know the answer to that one, better to be safe than sorry IMO).
When calling the dealers to get prices, ensure they are quoting or can carry it out under there 4+ scheme, some do and some apparently don't.