It has been my experience that there's only one water pump for an MB and that's the one sold at the dealer.
The last time I bought one for my MB(5 yrs. ago), the dealer got $100 + exchange. You have to return a pump with an MB Star on it for the core exchange, at least that's how it worked at my MB dealer. Some may offer more latitude with regard to the exchange unit.
After-market pumps and OEM quality pumps were all much higher than the dealer pump.
As far as "how-to", use the SEARCH facility. I don't know how much has been posted about 4 cyl. water pumps, but your engine is nothing more than 2/3's of an M103 engine and much has been said about them.