No compression in one cylinder on my '84 300SD

[Mamono]

First off, I want to say that I know nothing about cars. Any information mentioned is directly from a mechanic. I recently purchased a 1984 300SD. Before buying it I had a prepurchase inspection done in which they told me about a problem with one of the cylinders where it had absolutely no compression. The shop (MBI Motors in Portland, OR) tech did some further tests and told me something to the effect that it was not leaking from the bottom so it was probably just an issue of the heads needing rebuilt, a $1500 to $2000 job. He also assured me that the car was in no danger of dying on me, the only consequences would be poorer performance and possibly issues starting if the outside temperature was very cold.

Long story short, I went back to the seller and negotiated the price down to $1500 as I figured $3000 to $3500 wouldn't be a bad price for a 300SD.

I was talking about it to someone who works on these cars and he mentioned it might just be a stuck valve or something due to carbon build up. He recommended sticking automatic transmission fluid in the gas tank (with a full tank) which may break down the carbon and cause the cylinder to start working again. This coincided with MBI's diagnosis in that they mentioned that there was a slim chance the cylinder may clear itself and start working again.

My question is two-fold. First, is the suggestion of carbon buildup a possibility? If so, would ATF break down the carbon like the other guy said? If ATF is not the way to go, what would work?

Thanks