Re: which lower unit
You have a manual; that's the most important tool in your box of tricks. Read the chapter on lower units. You will need to "drop" the lower unit from the motor so you can replace the rubber impeller that pumps water. They get old and dry and must be replaced periodically (they are a "consumable"). While you have the lower unit off you should pressure test it to see where it leaks. The most common cause of water intrusion is simply the seals on the oil drain & fill screws. These should be replaced too (also consumables). Buy a half-dozen of these. There's at least a 50/50 chance that you don't even need to replace the internal seals. A couple 50 cent nylon oil screw washers and you're good to go.
With the lower unit off and the oil drained, put the oil fill screw back in with a new seal (nylon washer). Now take a tire valve stem and whittle it down enough so you can screw it into the open oil fill hole. Presto - you have a way to pressurize the lower unit. Submerge the lower unit in the bathtub or, if you have a nice bathtub, cover the lower unit with very soapy water. Use a bicycle pump to put about 10 pounds of pressure into it and watch for bubbles. It's just like testing a tire with a slow leak. Don't use more than 10 pounds of pressure or for sure you'll have leaks because you'll blow the seals. They're supposed to keep water out at sea level, not 20,000 leagues under the sea. Your unit should hold 10 pounds of pressure without any bubbles for hours. If it doesn't and you see bubbles then you will need to order and install a seal kit. No big deal since you're mechanically inclined. Post back if you encounter problems.
