Wow! This is what i was talking about - super helpful. Thanks for all the quick responses.

Couple of things to clear up - should have provided earlier.
Car has 110,000kms on the clock
A reconditioned box from the same guys that the BMW dealers use is $5k - got it quoted before I accepted the offer. My logic is that if I don't need a reco box I got a steal. If I have to do it, the I'm paying exactly what I was going to the first time around but I have a fresh box. Maybe my logic doesn't stick? Happy to hear the contrary view.
There is no opportunity to return the car again and to be honest I wouldn't want to - even with the box issue (potentially) I could sell it for more than I paid.

I should also mention that the car was fitted with a tow bar and an electronic braking package where I have this gizmo in the driver side footwell that help stabilize loads or something.