If it does check out that it is the O2 sensor, to avoid any problems I would get the original Bosch O2 sensor.

I have a 90 244 and a while back a mechanic spliced in a universal sensor, charged me $100 for the part alone (the universal one he used was around $50) and the check engine light kept on lighting even after resetting it. Long after it was changed to the original O2 sensor and it fixed the check engine light problem, the excessive fuel consumption and the black smoke puffing out of the tail pipe.

I checked eEuroparts and the O2 sensor you need is $149.