Diesels have come a long way over the past 20 years. You get more miles to the gallon, the price of diesel is only a tad higher than a petrol. The power output of a diesel has been matched to petrol cars now and then some.
I can't see anydownsides of owning a diesel anymore, they used to be clunky, loud and underpowered. This is no more.
Make sure the car is turbo diesel rather than just a diesel. Make regular oil changes with top quality oil just like any other engine and you should see thousands of miles out of the car and save some money and help the environment in the mean time.