As you mentioned, the dents are on panels that a probably from a supermarket carpark. As far a I can see, none of the dents are actually on the places where the owner of the car would be at fault, ok, maybe I see one on the door, but is that the driver's door? Also has the cambelt been changed, it's 5 years old and 55k miles, so perhaps you can negotiate with your dealer to also sort out /touch up some of the dings and small dents. You mentioned it failed the MOT, on what, brake pads and discs are replaceable and not un-comon considering the mileage and the big 2.0TDI diesel heavy lump up front? If there is no full service history and the somewhat higher price...I would say perhaps walk away, then again, if you have had a test drive, happy with it (listen for rattles), then it's up to you. It's a very good car for motorway muching...you should be able to get at least mid 50s mpg doing 70mph. Looks like it still has original Bridgestones on the rears? Perhaps time to budget for some nicer tyres for motorway munching that are not Bridgestones if you do get this car, and do fill Shell's / Esso's finest diesel for first couple of tanks to clean up the engine.
Regarding if it has crappy tyres on the other axel that is not Bridgestones, some just do it to meet the min requirement to trade in a car, if the crappy make tyres have plenty of tread left, this may be the case, if it's low tread, then yes as other say, the previous owners didn't care.
55k miles in 5 years isn't excessive, it's actually around 11k a year, which is below what a diesel should be used, as most would say, below 10k, go petrol, above 10k go diesel...afterall motorway munching isn't that stressful for the engine compared to city driving..