Cleaning and de-greasing is important. I use grease cutting detergents like Simple Green (concentrate from spray bottle) or Dawn (diluted) with brush to do the cleaning. Then a rattle can self etching primer that is intended to be top coated. The dark blue gray paint from the filling station can be sprayed or brushed onto the cast iron and it will look just fine. If the valve cover or oil pan it is best to spray it as brush marks will show. The primer will help the paint bond and last much longer. I have some that still looks good after 20+ years. I have used a piece of cardboard to prevent over spray. Taping with a couple of layers of newspaper or paper can also be used. If a little paint eludes the barrier a little paint thinner on a rag or paper towel can remove it. Adding catalyst to the paint helps it cure quicker and also longevity. It means you have to remover over spray quicker.

It's all a question of how good a job you want to do. One end of the spectrum is, clean it all up as best you can, cover what you don't want painted and Rattle Can away.

On the other end of the spectrum, I would recommend removing all the engine sheet metal and clean and paint all that separately. You can do all that yourself or send it off for powder coating. Either way, while all that is off, take this opportunity to clean everything you can on the inside of the engine. There are lots of great advice on the best ways to do that so I won't go into the details but any sludge you remove NOW is just that much less that'll be floating around later on.

This might also be a good time to replace all your questionable hoses too. Again, you didn't really say how thorough a job you want to do. If you're painting just to unload it then maybe don't bother. If you're painting it for your own satisfaction then do a really good job and it'll be a source of pride for many moons to come. And probably a bit more reliable too.