Everyone is going to crack on me but don't do it, maybe if you are a collector and working to restore a old boat with it. but if you want to just fish, ski and be a boater. Life is too short. Used motors are kind of strange, if someone is selling it there is a reason, it has been trouble for ever. One that old you will have major problems with getting parts. Not worth $0.50 to me that is.
I heard they have some problems with the electric shift on some of the 69-74 models.
I have a 1976 I paid $700 for it.Parts a readily available for this motor.would check and make sure they are for that one too before buying good luck.
I'm working on a 1956 Johnson 10hp. All parts are readily available. Johnson and Evinrude parts are usually available, Mercury are but may take more looking and $$$. If not new, tons on E-Bay used.
As far as what it is worth; that decision is up to you and the seller.