The M103 engine has a ratcheting tensioner which makes the chain virtually trouble free. Conventional wisdom holds that these particular engines do not need the chain replaced for preventive maintenance purposes, only when the engine is apart for some other reason.
It sounds like you already have it apart, so here's the drill. To begin with, ensure that you remove the tensioner plunger by pushing it through, then feed it back through just enough to put it together. If you were to leave it in place you would probably break something when putting the new chain in place.
Once the new chain is in place, use the special sealer from MB for the upper front timing cover. This sealant is slick and allows you to start the shaft seal, then push the cover rearward without disturbing the bottom U seal. Gob the sealer in the bottom, rear corners. If you do this properly it will not leak, if you don't, it is guaranteed that it will leak.
Best of luck,