If you haven't already ordered your car, I'd just add the Assist. I believe enhanced usb / bluetooth streaming + possibly other features can be added via retrofitting the appropriate combox. But it'll cost more than simply ordering the Assist option.
2015 228i M Sport + Track Handling Package
BSM / Coral Red / Aluminum Hex
Tech / Premium / Lighting / Heated Seats / HK / Enhanced USB+BT