Putting the universal stuff in with the BMW coolant is okay. Its better than having low coolant by any stretch of the imagination. When you diagnose and fix your leak, replace all the coolant with one kind and stick to it.
As far as your part replacement costs, I have seen cooling refresh kits go from $250 to $450 (without the radiator). Difference in price is based on including the fan clutch and temp sensor. At the very minimum, get the expansion tank & cap, the water pump+gasket and thermostat+gasket. Those will cost you around $175 or so. Stick with the plastic thermostat and housing and make sure the water pump has a metal impeller instead of a plastic one (you will see them listed as such.) Have someone check your top and bottom hoses before you order or buy parts.
If you have a garage put on the parts, expect 2-4 hours of labor, plus supplies (coolant.)