Oh you're in for a treat (sarcasm). The plug is about the size of a dime with a hex socket in it and located on the drivers side of the block below the exhaust. Pretty sure the only way to get at it is by removing the wheel well liner. You'd first remove the front left wheel then the liner is in two large pieces and you only need to remove the front half. It's held in with a bunch of plastic rivets that will need to be replaced.

I'll try and look up the capacities later but if I remember correctly draining the radiator and expansion tank is about 2 1/2 gallons and about another 2 for the block.

^^ those are mixed values. The BMW fluid needs to be mixed 50/50 with distilled water.