The maintenance "warranty" that BMW offers is 4 years/50k miles, whichever comes first. It includes oil/filters, microfilters, brake pads/rotors/fluid, wiper blades, and engine belts. The interval between services varies depending on how you drive your car, but tends to occur every 9-12k miles on average (depending on worse/better driving conditions, the service countdown will be faster/slower). Costs for service will vary depending on which dealership you go to; there is no set price for all dealers (labor/parts prices may vary). They won't have exact price quotes available now, but call up your dealer and ask how much it is for an oil service, inspection 1, and inspection 2 for a 335 (the order of service goes oil service, insp 1, oil service, insp 2, oil service, insp 1...) Add up four or five of those costs and that should give you a rough ballpark of how expensive a 135 would be to maintain for four years.
BMW oil is 5W30 synthetic (M cars use a different "motorsport" grade). BMW will NOT pay for an oil change every 3k miles; it can only be done if the car is within 500 miles due of service per the indicator. Anything above and beyond that comes out of your own pocket.
Again, this varies from dealer to dealer (depending on how nice they are), but I do not believe clutches are covered under any factory warranty (if they are nice, they may goodwill some or all of the cost).