I have looked at injector replacement in a lot of detail before (not Merc). This is a quick summary of what I discovered on the pros and cons for new/recon/reman.
If you are planning to keep the car, buy new, and try and find the best price which might be for example a Bosch unit new, but likely not from Mercedes. Might also need coding. You will need to do some homework to find which part and what's involved if you want to minimise the cost.
If you are getting rid of the car, it is suggested recon/remanufactured. You need to find a company that can be trusted, as recon is replacing some seals, and remanufacturing is generally more thorough, but the reality is that none of these processes will return the injector to as-new condition. Therefore there is a school of thought that you are better off not even spending your money on a recon/reman unit, and just trying your luck on a second hand unit known to be working when removed. Much cheaper. A trusty garage to do this for you is likely worthwhile unless again, you are prepared to do the homework around which parts required and exactly how.
Hopefully someone will be along to advise on more detail of the specifics for the w211.