You do want to go fast enough to create a bit of a bow wave because it means its pushing the water away from the vehicle. You just don't want to charge in. Your more interesting challenge with fording rivers in Africa is it depends where as there are significant parts where you probably aren't going to want to wade through first because of the risk from local wildlife. The only thing you can do then is to sit and wait to watch another vehicle going through and see how far up it comes. Your other key factors are how solid the bottom of the river is in terms of traction and also whether there are any rocks / logs etc that you can't see underneath the water.

Your key issues are going to be your air intake, you spark plugs / lead, electrics in general. The key one is your air intake because BMWs often have them quite low down. In respect of diff breather pipes, etc, if you were properly preparing the vehicle for doing this alot then it would make sense but given you are only planning wading in exceptional circumstances, I wouldn't bother. You just need to be mindful that your diffs may require an oil change at some stage after because water may have got in them. Its not an immediate issue that has to be immediately rectified but it would cause damage over time. Have fun