If your car is made before 6/2000 it would have 001 540 08 17 and 001 540 10 17 sensors for front sensors. They are both listed as $110 MB list. Both of these numbers aply to vehicles without code 801 (which I don't know). They are also for US cars so you could have different ones.
The O2 sensors in front of the cats measure the amount of O2 left in the exhaust after combustion. This tells the controller the instantanious mixture and the controller alters its calculation for fuel based on this. If slightly rich it removes fuel which eventually causes too lean and it adds fuel. This "closed loop" control keeps the mixture at an exact mixture with the exact fuel to air mix that results in stoiciometry: HxCx + O2 = CO2 + H2O.
The act of running slightly lean and then slightly rich gives the average above. The extra O2 dduring the lean swing is used by the cats to do the oxidation reaction.
The rear sensors are another story.
Bosch Master, ASE Master, L1
33 years MB technician