The lower gearbox needs a special grease. There are two screw caps that need to be removed, one higher up than the other. Then the grease (a special very thick oil) gets forced in through the bottom hole until it comes out clean from the top hole. Some water will likely come out too. This needs to be done at least annually. If significant amounts of water come out, you will likely need new shaft seals and a bearing inspection/replacement.
There is a rubber impeller water pump in the top end of the lower unit that the blades eventually break off of. If not replaced in time the motor will overheat, leaving you in that remote fishing hole for a very long time. Replacing or inspecting it is a project usually left for a marine mechanic. In my experiences, it has been under a threaded cap in the top end of the lower unit. The lower unit needs to be unbolted and removed from the upper motor section. There will be a cap with a hole in the center for the long shaft that comes from the motor. The impeller pump will be under this cap, which requires a special spanner wrench to remove. Since it's aluminum and the housing is also aluminum, they frequently corrode together, requiring all kinds of work and special words to get the cap unscrewed.
For both of these projects, it's best to hire a marine mechanic for the repair work. Since you know very little about the history of this motor, it might also be best to have the marine mechanic go through the whole outboard motor top to bottom to check and fix whatever might be needed. They require a lot of special tools and skills to service properly, but the lubrication and water pump are the two items that are most likely needing attention.
If you decide to paint the lower unit, be sure not to get paint on the anodes. These are zinc alloy pieces that are attached to the lower unit to prevent electrical charges in the water from corroding the lower unit. These will very gradually be eaten by these electrical charges instead of the aluminum of the lower unit. If they are covered with paint they will not work. They may eventually need to be replaced if the outboard is left in the water, but will likely last the life of the motor if the motor is removed from the water after each use. Just remember not to paint or remove them, as they are very important.
Boats can be a lot of fun, but the maintenance is costly for the amount of use you will get out of it. I've owned boats up to 26', with inboard, inboard/outboard, and outboard power. I have a 21' Sea Ray cabin cruiser with a Chevy 350 I/O drive sitting in my driveway. There is a sign on the wall inside the cabin that reads "A boat is a hole, surrounded by water, into which you pour money", and it's a very true statement, no matter what size boat you have.
Charley