I drive a 318i, and that gets around 34-35mpg, give or take.
the 325tds gets 38mpg, but with the increasing price of Diesel fuel, i would say it works out about the same because youll pay 79.9p a litre for unleaded. but 85p a litre for diesel, balancing the figures out.
The TDS is the model with an intercooler, which gives it more guts so i belive, the performance 0-60 is about the same as the 318i as well, but im guessing the diesel has more torque, from lower down the rev range.
As for the servicing side of things i dont know the intervals of servicing for the 2.5 diesel, nor am i sure of the cost, but i do know that a properly maintained BMW engine will last you for pretty much as long as you want to drive it, petrol or diesel.
At the end of the day a BMW is a BMW, and they are all great cars.
out of interest how many miles is on the car?
and make sure its got a full service history! the last thing you want is to buy your car and then something shitty goes wrong and you foot the bill