I don't know about US, but 03 X5 4.4 came in 2 different engine.
M62 or N62(later one). By just looking at it, the N62 engine has relatively newer engine cover with 2 lumps, one on each bank of the engine.
Later one is easier to change, it's just a ordinary alternater.
However, the older one, being a water cooled alternater, it's a bit harder. And require a coolant top up and bleeding of the coolant system.
But, seeing how cheap you got quoted for the alternater, I believe it's the later one you've got. Coz the the older one is like 2k part alone.