I like Dupli-color Semi Gloss Black for paint. Prep all depends on what you start with AND what your goal is.
My projects are for ME alone.....not a restoration, just a good looking model train. Body needs to be smooth and grease/oil free. I have used 'aircraft' paint stripper (use with caution) to get a clean surface but if paint is stable and not chipped to a level to hurt final coat....go with as is. 100 different repaints will take 100 different solutions.