How many miles did your '87 have on it when the guide broke? I have an '88, 420SEL, with 75K, and just had the chain and tensioner replaced, but not the rails (the mechanic's decision). This was done at a Mercedes' service center, and the service consultant said the guides (rails) are not actually plastic but a composite of some nature which I don't recall, and that the material is actually very durable. He said, further, that the guides on my car showed no wear. I'm still worried. I'm the third owner, and I supposed the guides might have been replace earlier but I have no record of it. All that I have read indicates that the 4.2, V8 is at high risk for chain failure so, as other have said, replacing the chain and guides is cheap insurance. With only 46K miles on the car, though it's a shame to have to lay out that kind of money. This sort of thing ought to be on a Mercedes recall and done at their costs, in my opinion.
Jerry