When you say Model 1- is this the older, 1930's model Co-op, not the co-op versions of the Cockshutt tractors beginning in 1946?
Go to "Tractor Photos" on the left, and put in "Co-op" in the search, with the quotation marks. On about the third page are several old Co-op tractors- Models 1, 2 and 3. Take a look and see if that's what we're talking about. If its one of those, in good running condition and good appearance, I'd think 3K is in the ballpark- they're a much rarer tractor than the E3 that rrlund is talking about.