mileage can be a hard one to guess tbh since you dont know if its done 120k easy miles or been thrashed with in a inch of its life, just make sure you have a good look and listen failing that get the AA to do a check or a BMW garage.

i have worked on cars with silly miles and some with hardly any, if it doubt walk away or try and get a price where you could still have enough to cover any work needed.

With regards to the car your wanting at that mileage you will want to change oil and filters in the box if never changed and change the oil in the diff. see/ask whats been dont and haggle from there.