If you want reasonable running costs and performance, the 2.0-litre diesel is the only sane option.
The 2.0-litre petrol engine can struggle with the weight of the car, and you'll be hard pushed to get more than 30mpg on average. The 2.5 and 3.0-litre petrol models are faster but even thirstier. The 2.0 diesel averages just under 40mpg, with the powerful 3.0-litre diesel managing about 35mpg.
The X3 range was revised in '06, when the ride and the look and feel of the interior were improved. The exterior got a subtle restyle, too, so these later cars are the better bet.
An entry-level model was also introduced, but the SE is the best option on the used market, with a decent amount of kit as standard. The Sport model adds different trim, wheels and firmer suspension, but ride quality suffers.
An automatic gearbox is available with only the 3.0-litre petrol and diesel engines.