1) The 1995 FSM recommends using 80W-90 gear oil. I assume there is no difference for '93. When I filled my differentials, I actually used Lucas Oil Stabilizer. It said on the bottle you can use 100% capacity in differentials, and it has worked out great for me. Also, if you have a limited slip differential, be sure to add 4 ounces of friction additive. As for capacity, its 2.1L, but instead of worrying about measuring it out, just fill it up to the bottom of the fill hole as long as you're on flat ground.
2) I would start by scrubbing it down with car wash soap and a green scotchbrite pad. This will get the oil and dirt off of it as well as start scuffing it up. Then, use 220-320 grit sandpaper, or if you have air or power tools, use a roloc fiber disc. I would recommend just spraying it with black rustoleum spray can paint.
3) That rubber plug is probably dry rotted. Replace it when you get the oil and while you're prepping it for paint, be sure to clean up the hole.