Help! I need some advice.

I bought my 560SL last November and have been poured about $2K into it already.
I have replaced both fuel pumps, fuel filter, spark plugs, wires, distributor cap and roter, the front flex disk and front sub frame and motor mounts. basicaly the car runs strong but I don't know when the timing chain was last replaced (the car has 120K mi). So I took it into an independent shop this morning to have the chain, upper guides-rails and tensioner replaced just to be safe.
Well.... The mechanic just called and told me my cam are showing wear and that I will need to get them replaced soon at the sum of about $1500. My question is if the engine run's fairly smooth and I don't hear alot of clatter from the lifters, how long do I have before I have to replace the cams. I only plan to drive it 3000 to 5000 miles a year and will change the oil about every 2K. Also how much of a job is it if I do it myself?

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Hud

88 560SL
97 E320