The reason you want it diagnosed is there are before and after catalyst sensors on the X5 (4 I believe total). Each costs roughly $75-$85 USD. As far as special tools go, I haven't changed one on a X5 before but usually standard mechanics wrenches will do the job, however some O2 sensors have the need of a special tool. Just go under your X5 and take a look before and after the catalytic converter.

Plus, if it is the O2 sensor, judging by the age of the ride it will be nearly impossible to get off. And if you do all that and it wasn't the O2 sensor, you've spent a lot of time and money on the wrong problem. Maybe you can find a cheaper BMW specialty shop to diagnose for you

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