I have purchased a 1986 300E with a 5 speed that is also in need of a valve job. My mechanic said the price for him to do seals and guides would be $400-600. If the cam is bad, (which is common with the early 3L), then the price could go to $1000-1200. There is an updated cam which means chaning lifters and maybe rockers. I suppose the usual timing chain, water pump and belts would be advised if these haven't been done before since they all have to come off.
Dave, I have heard that the 5 speed is quite rare. I am interested in the number produced. Would you know the figure?