I had an '87 560 SEL with the same problem, my was started as an intermediate rough idle and changed to continuos rough idle. Recommndations that people gave me were as follows:
Check for vacuum leaks
Techron fuel injector cleaner
Change plugs, caps and rotors
Change ignition wires
Replace fuel injectors (irritating job, rubber parts, crack like thin walled glass)
Then when all these failed, and they did all fail, I took it to the dealer and they told me the fuel distributor was defective and needed to be replaced
$1,800 plus labor plus tax. I sold the car, not because it was a bad car, I just couldn't handle the bad idle, and was not going to put over $2,000 in a car that i paid $2,000 for in the first place.
I replaced it with another 560SEL, this time an '88. This one as well once it hit 127,000, same bad idle, replaced the same parts as before no change again, sold it againbecause the rough idle was driving me nuts, and replace it with a 300SDL.

So I can suggest you start with the vacuum leaks, then move on to , the fuel injector cleaner, you've already done the caps and rotors, well there's the ignition wires, then the fuel injectors. Then theres the possible diagnosis of the fuel distributor. My advice, don't fall in love with the car, and don't let it bankrupt you, there's alot of these out there at reasonable prices. If the idle bugs you and you can't fix it, sell it!

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jcciem
'94 SL5000 60,000
'91 350SDL 545,000
'91 350SD 197,000
'00 E55 AMG 77,000