To rebuild at home [calipers], buy caliper rebuild kits at the dealer (I recommend doing both sides--stopping in a straight line often has its advantages). Get new bolts. The bolts that attach the calipers to the car come with 'loctite' on them and MB recommends you replace with new ones each service. Also get the grease that goes on the back of the pads to prevent squealing.
On the way home, stop at a McParts and buy a spray can of Brake Kleen (both the previous names are generic, any brand will do) and good quality brake fluid (I recommend Castrol LMA). While you may use Mercedes Benz Brake Cleaner (I don't know if they sell it, but it would probably be expensive) and MB Brand Brake Fluid (Pricey but very good), don't use aftermarket caliper rebuild kits or bolts (the MB parts are high quality and not very expensive).
The job is a bit messy; I would use an old shower curtain or something to protect the drive; have plenty of rags. Remove Left wheel and then the caliper. The big problem will be getting the stuck piston out of the caliper. Often, clamping the free piston and applying the brakes to force the stuck side out, works. Be careful, there is LOTS of pressure there. You may have to move stuck piston in before it will come out. Be inventive--but careful to not damage piston or cylinder in the process. Do not Ever separate halves of the caliper assembly.
Once you get pistons out, check for grunge in the cylinders. You may use light emery paper to remove grunge but if there is pitting, you must replace the caliper (now you can see why yearly or bi-yearly brake fluid changes make sense). Clean the notch in the piston where the new seal will go. Clean everything well (ride your bike back to McParts for more Brake Kleen, if necessary). Lubricate with a little brake fluid (just the pistons and cylinders; don't slop any on rest of caliper, don't want it on brake pads) and put on the new seals.
Press pistons all the way into cylinders. Put a very thin coating of MB Brake grease on BACK of pads and metal sides where they rub against the caliper. Be very careful not to get any on braking surface of pad (Brake clean comes in handy if that happens). Install pads and put caliper back on, using the new bolts.
Next tackle the Right wheel. It will be much easier with no stuck piston to deal with and by the time you are finished, you will have sufficient knowledge, experience and confidence to open your own BrakesRUs franchise. Plus, your car will stop flawlessly for as long as you own it (providing you change fluid regularly from now on).