I have a 1979 280SE that I recently purchased from an auction for a couple hundred. It's in good shape, except the engine. What I am hearing (how to word this) a robust knocking when at 2500 rpm or above, but levels out with no load. Now, heres a little background information.

Has 213,000 miles.
Doesn't overheat.
Oil is very clean. (no water, particles)
Engine runs good (strong accell., quick responce)
Have changed gas to premium, added drygas.

Symtoms:

At start black smoke, but clears out when warm.
(Also very smelly...)

Engine missis periodicly.

Number 1, 3 have oily spark plugs. (There's alot more to this one. I had recently changed the spark plugs, and found them in normal condition. I changed them and found the knock had gone away, but I ran it down the highway, and in about 2 miles it started knocking again. The next time I checked the plugs they were oily)

The idle speed was set @ 1800 RPMs, now I had lowered it to 800rpm, and was successfull. But when I did, it ran for about 20 mins and stalled, and didn't start back up... so I reverted back to the setting and it started.

Oil pressure is high (as far as I know that's Ok)

Hard to start when hot (running for more than 30 mins)

The engine is overall pretty loud.. possibly lifter noise.

I have not checked any timing, compression (due to lack of funds). But if anyone possibly may have an answer, that would be great. I have found a ton of info, but I have so many variables involved that I overwhelmed. My thoughts were possble heavy deposits on lifters, or advanced ignition timing off... maybe running rich??? (due to oily/gassy smell from exhaust)
Thanks