Engine cover, I know you have a V6 so I cant speak for the exact texture, but sanding is up to you on textured (rough) parts, and is recommended for smooth already painted parts. The point of sanding is to give the paint a rougher surface for it to adhere to.
If you are painting an already painted part of your engine cover, go ahead and sand it lightly, you dont need to remove all the paint, just scuff it up. Heres a picture or two of my engine cover, you can see when it was taped off, that I masked off the rough black areas, sanded the silver parts lightly, painted the silver parts red, then clear coated the entire thing.
http://www.camaro5.com/forums/showthread.php?t=223456
As for pinstriping the wheels, clean with soap and water first, the clean the area the vinyl will be applied to with alcohol and a rag/lint free cloth, or even a paper towel as long as it doesnt leave lint. Here is the DIy for pinstriping wheels:
http://www.camaro5.com/forums/showth...18#post4937918
Alcohol removes grease, oil, contaminates that might prevent the vinyl from sticking. Also, its easy enough to do that you do not need to do the "wet method" where you wet the wheel with water to allow the vinyl to slide around, you are better off just getting it on there dry.