Re: '71 Mercury 650 65hp Outboard
Props are usually specific to a mfr, ie., Merc, OMC, Yamaha etc. There are three specs that are important to match to your motor/boat. They are # of splines on the prop shaft (yours are probably 15), the overall diameter of the prop and lastly, the pitch, or how far the prop would travel in one revolution (theoretical).
If you look on the back face of your prop around the center hub there should be a series of numbers. The last few digits will describe your pitch, ie., 17P means 17" pitch. The numbers may also be stamped somewhere on the body of the prop.
50:1 (50 parts gas, 1 part oil) is the correct mixture. Use any name brand MARINE 2 cycle oil rated TC-W2 or W3.
Again, lower unit oil is MARINE grade SAE 90 extreme pressure (EP) gear oil. Lots of posts here describing how to do it.
Water pump is a vane type impeller that is driven by the driveshaft inside the waterpump housing. You need to replace at least the impeller with the correct one for your motor. Size of the impeller changes with different models and HP. If in poor condition, you may need to also replace the upper pump housing and other seals. No way of knowing until you get it apart.