I'd agree that I would have someone else do the alignment. This is ONE place where the dealer might just be the best place. MB alignments involve some unique steps that most shops won't know about, like using a special bar to put lateral pressure on the front wheels, etc.
If you get the tires from Sears, I would recommend you get them balanced at a shop which uses the "official" MB 2-piece wheel weights. The cheaper, conventional 1-piece wheel weights WILL come off and leave you with unbalanced tires after a period of time (sometimes a very short period of time)...