I did the conversion on my 71 220D this past weekend. The car has never started so easily! Of course, this is also due to a long overdue valve adjustment, but I am amazed at the difference.

The 220D is a four cylinder, so I didn't have worry about the extra cylinder stuff. According to the instructions for the 5 cylinder, you will need (obviously) an additional glow plug and a bit more work. I didn't see anything about need an additional relay. I would forget about the kit on eBay and contact Rusty at mbz.org. That's where I got my kit, and no doubt they will be able to sell you whatever else you need to complete the job.

Eric