I've seen early 450SL's with everything worn out and some that were like new so age is only one factor.
The bottom chain rails last a very long time and on't wear out very quickly. The top ones can crack and should be replaced if they appear to be worn or broken. The chain tensioner is something I wouldn't replace unless it fails.
You can make your own pin tool without too much effort. The tool to replace the master link is far to exspensive to justify purchase or even rental. I grind the ends of the pins off and use two hammers, one a ball peen, to rivet the ends of the pins after assembly. I've never had one fail and it costs nothing since I already have the hammers. You will need to press the end plate on over the pins with a pair of vise grips - not hard to do.