I recommend keeping some of the narrow black fabric covered hose that fits between each fuel injector in the trunk in case you spring a leak. I also replace the stuff about every five years - it's cheap enough, why not?
Also, I'd suggest keeping an extra plug for the last fuel injector in the glove box. If this thing starts leaking on you while you're on a trip, you've got a serious problem and I think it unlikely you'll be able to get a spare easily.
Finally, I recently replaced the opaque plastic fuel return tubing that runs around the engine of my '83 300CD-T. After 223,000 miles, it seemed like it had discolored and was a bit brittle what with the high tempteratures and age. These items came from M-B as they have the correct fittings on the ends.
My suggestion for all of the above is to acquire from M-B. It's all original equipment specified for your car and (my opinion only here) it is reasonably priced and certain to be appropriate for your application.