The block drains have copper or aluminum sealing rings, its a good idea to replace them. the radiator drain has a built it seal and is re-usable. make sure the car is flat on the ground when you drain it, so raise it off the stands and set it on the ground for a few minutes. If the car has been properly maintained all you need to do is drain and fill, the residual amount of coolant left in the heater is fine. I have actualy known techs to save old coolant on well maintained cars to top up coolant on other cars during oil service. If the coolant is discolored, flush it with clean water, heater core and all.
If you flush the system, add 1/2 of cooling system capacity of pure coolant, then add water, this way you wont end up with weak coolant from adding50/50 mix to water.
If you don't flush, fill with 50/50
If the block doesnt't drain, poke a pocket screwdriver into the drain hole, its probably blocked with detritus, this is not abnormal.
My method for filling BMW's may be a bit different than others, remove the upper hose (left side hose) and stick it on the radiator, fill the radiator, then remove and stick in on the engine and fill the block, now put the hose back on as intended. this will ensure the cooling system has enough coolant to bathe the T-stat and eliminate the biggest air pockets.
The radiator drain is very very low torque, it takes very little torque to make the treads catch enough so it won't unscrew.
I always test drive a vehicle before I service it, so I flush them at operating temp, but for safety you should wait until you can lay your hand on the side of the engine block comfortably.
The proper amount of coolant is in your owners manual, use this information before a non BMW source(reprinted with BMW permission is non BMW).