1984 380SL price?

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Am considering buying a nicely maintained, 1984 380SL with 135K miles. It has a battery issue when it sits for 2 weeks without driving, (the current owner has an adapter so you can plug it in to a wall outlet for a slow drip of voltage, which solves that problem) and there is a sound when you turn sharply to the left or right coming from the steering wheel and the antennae will not go down all the way. Other than all that, it seems to be in great condition. Any advice on a proper offer price? Or insights into the votage issue?