It sounds like you made the smart decision to buy the Starmark program, none of those items are really out of the ordinary (the radiator neck and t-stat are still ok??). Remember that some Starmark Inspections are more thorough than others....
Sell the car and buy a ML?? Wait a few more years, any and all vehicles are better after they have 'aged' a little, personal opinion (and not that of Daimler/Chrysler) is that you would continue to display your intelligence by hanging onto your 1991 300TE 4-matic and enjoy it! 1 quart of oil every 3,000 miles is excellent! Keep changing the engine and transmission oils as recommended in your owners manual.