Worth double-checking that the car actually has a Euro 6 engine as 2015 sounds early for Euro 6 (although I have seen a few 2015 W246 supposedly fitted with Euro 6 diesel engines). If you are buying a diesel car, easiest way to verify this is to check it on the TFL website to see if the car carries a diesel surcharge when driving into Central London.
Secondly, and this is only my own (unfounded) opinion - the A and B classes are not build to the same quality standards/longevity as the C and E classes, so at 145k miles, I would consider this to be very high mileage and you may begin to see issues that might inflate your maintain costs.
I have a couple of 'high mileage' W169 A-Class diesel cars (similar to the W245 which is the predecessor to the W246), and while the engines are going strong we have experienced many issues with the springs, struts, glow plugs, air con, etc that significantly increased the maintenance costs.