Brake Pad Sensors

[Spanky Deluxe]

I'm planning on installing new brake pads soon, likely next week, since my low brake pad light came on a couple of months ago (but I haven't done many miles in between).

I've already got the new pads, front and rear, but I think I need to get some kind of new brake pad sensor(s) but I don't know what I need. Do I need a sensor for each wheel, are there different sensors for front/rear and are there different sensors for different model years?

Our X3 is a late 2004/early 2005 3.0i Petrol. Also does anyone have any experience with this? My automotive engineer friend will be helping me do it and he says that some car makes are a nightmare while others are as easy as pie.