Hello,
I have also used Mercedes rebuilt water pumps for around $100.00 with a discount, and have had no problems.
The fact is Mercedes-Benz does not make their own water pumps. Some aftermarket suppliers have quality after market parts and may even carry the original manufacturer's pump. Most reputable suppliers will not sell junk. Consider the options. The Mercedes rebuild is the best value for OEM quality. If the rebuild was not available I would consider an after-market pump from a reputable source for a savings of over $300.00.
The best way to make any water pump last longer is change coolant on a regular basis and do not leave the car inactive for long periods of time.
I like to use some permatex #2 or gasket shellac on the water pump screws to seal the threads.