While there's no harm in replacing the timing chain, the general comments from techs in this forum is that M103 timing chains don't fail if treated to frequent oil changes. There are very few incidences of M103 timing chain failure in the archives, and then it's usually caused by a foreign item. The ratcheting tensioner when working ensures that there isn't any slap.

Checking chain stretch is a primarily Diesel thing. The ratcheting tensioner makes such a test on a M103 not very meaningful... or so I understand.

If you're removing the timing chain cover to replace the gaskets then by all means replace the chain and guides and maybe even the tensioner.

If you can do the timing chain cover gaskets yourself you might as well do the head gasket as well. Same level of difficulty.

If your engine leaks now, it will continue to leak with Mobil 1. It shouldn't leak with Mobil 1 after you replace the head and timing chain cover gaskets.

Sixto
91 300SE
81 300SD