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The plastic guides can't really be judged by visible wear. They turn brown and get brittle over time. My tensioner arm had worn grooves. The danger is that the chain will be loose (either from the hydraulic tensioner getting old or the chain stretching), the chain slaps the brittle guides, the guides break off a chunk, the chunk gets wedged inbetween the chain and gear, the chain breaks/skips, and the pistons hit the valves, the valve cover breaks, the cam bearings break, etc. Very ugly and expensive to repair.
From what I've seen on this site, 100K seems to be a good mileage to do this. Your mileage is getting to the scary zone if this has never been done. Although I had 88K when I did mine, the car was 19 years old (like yours) and I did determine that the hydraulic tensioner was definitely not up to snuff after removing it and comparing it to a new one.
A "timing chain job" is not really a difficult job (I'm a data analyst and I did it) but it is lengthy, tedious, and not tolerant of mistakes. Some specialized tools are very helpful in order to accomplish it unless you are very resourceful. There is a lot of information on this site, and in the MB CD manuals. Usually, the tensioner, chain, upper guides, and tensioner arm are replaced as a set. This may be overkill. I analyzed this stuff quite a bit and I suspect that the chain gets blamed for stretching or breaking when in reality it is stretched or broken only AFTER the other things above have occured, and the heart of this problem is the tensioner itself.
While you are doing this, valve cover gaskets, oil tube connectors, all belts, and maybe even the water pump/thermostat/front hoses are good to do while you are in there and you don't know the service history of the car.
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