I recently had the timing chain, tensioner and top rail on my '88 420SEL replaced for $360 labor. I supplied the OEM chain, tensioner, rails and valve gaskets which I got myself for $165 total and the independent agreed to use. I had 77,000 miles, but had recently bought the car from an older couple and it had not been driven much in the last several years (i.e., the low mileage).

There was a little bit of play in the chain, so they went ahead and replaced it (I was also having the smog pump replaced and asked them to check the chain) and the top rail. They said that the top rail showed a little wear but that the bottom guides were fine, so they did not replace them (I had supplied the bottom guides as well -- a total part cost of $6).

I was a little annoyed that they did not do all the guides since I had gone to the trouble of getting them, but they assured me I had nothing to worry about since the top is the trouble-maker (and based on your post, saved me a couple of hundred in labor costs). These folks come well recommended and seem knowledgeable, so I will go with their recommendation and, like you, just do the bottom at a later cycle or when something in the way has to be dealt with anyway.