California gear box speed sensor

hi guys I am a bit worried, I put a deposit on a 2012 model last week with 31,000 miles, when I got home a noticed a 2011 model near me so I went to see, its got 18,000 miles, the 2012 model has some slight marks on the paint , all the wheels needs doing which he agreed to do ,he has receipts to show that the gear box speed sensor has been done at a cost of around 7k, the 2011 model is mint all over, but the gear box has not been done, the chap selling the 2012 model is saying I am stupid buying that other one because its only a matter of time the gear box will go, is he correct or just trying to get me to change my mind and buy his
can someone put my mind at rest please, do these gear boxes all pack up in time like he says