There are some previous posts on this and they are the basis for my answer (I'm not a pro).
What I remember is, when you adjust the valves, you can check to see if you need a new chain by how many degrees of stretch are indicated. MB recommends 3 degrees is the limit but some of the pro's say 5 degrees.
I also seem to remember the rails usually do not need replacing. Howewer, if it were me, while your in there anyway, might as well replace them.
Anyhow, do a search or wait for someone who really knows what they are talking about to respond.
84 300SD (sold)