I've been tinkering with an 87 300SDL that is a lot like yours on start up so I am interested in this thread. The SDL smokes quite a bit on start up and an intermittent loud metallic tapping sound. The SDL has 254K on it and I think it has the original injectors on it based on the service records that did come along with the car. I am considering the injector rebuild route myself.

One of the local members in Chicago has used Greazzer on this board for injector service. You may want to check that out. Here's the testimonial.

http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/341985-word-about-greazzer.html