[pwrdbym]

Good on you for doing driveline fluid changes, something 99% of owners don't do. The front, rear, and TC all have fill plugs and you'll top them all off (full when fluid runs out of fill). The front diff actually has a drain plug.
Front and rear will take about 1.2-1.4 liters and the TC is less than 1 liter.
There are alternatives to the BMW stuff, such as Red Line 75w90 in the diffs and the Shell TSEC for the TC.
I didn't replace my plugs, but that's up to you. Assuming they're in good condition you can use your judgement to reuse.