Well, here's my 2 cents. I had the valve seals done on my '86 300E with the 103 motor. Oil was burning about 1 qt. every 600-700 miles. Seal replacement was $300 at independant. Oil usage went down to about 1qt./2000 miles. It's been holding steady for almost a year now.
At 256K, the head has never been off, as far as I know. According to most posts here, the chain is not an issue, even at high miles. Is it worth it to replace just the chain guides as preventative maintanance?