I just bought my '77 550k last weekend. Paid $700 for it, which may turn out to be a great deal or could be a couple hundred too much. The last owner bought it in 1980 with 7k miles, then a year years later had hip surgery and parked it in his garage with 7,500 on the clock and there it sat until I bought it from his grandson 30 years later. It's completely original including the tires, 4x4 exhaust, seat etc.
But as good as it seems, I figure I will spend about $300 getting it on the road with a new battery, tires, new gaskets in the carbs, fuel lines, brake lines, etc. I am keeping my fingers crossed that the exhaust isn't just rust with a really nice chrome finish.