2014 bmw x4 gearbox

[Bert345]

Hi id really appreciate some advice on bmw x4 diesel 2014 about 60k on clock belonged to a friend. It's just passed nct and next day gearbox went. Quoted 12k by BMW to repair it. I can buy it now for 5k and am looking at getting local mechanic not dealer to repair it. Just wondering if anyone has experience with this issue-does it usually lead to bigger probs? I'm looking at gearbox from crashed x4 for approx 2k but then if I get reconditioned I'm being told 5k- are reconditioned worth it?
I'm not clued up on any of so any steer from ppl in the know would be most appreciated
Thanks
Roberta