Any parts that you can, use a belt sander with an aggressive grit (start around 100 and see how that goes; adjust up or down according to results). No worries going to bare metal. Use any other heavy grit sander on any other parts it can reach (they have lots of small specialty sanders these days). On inside corners and other hard to sand places, clean as well as possible and then use a rust remover (brush or wipe on) that will also turn into a primer - available at any self respecting hardware store. Wire brushing is good only to remove heavy, thick rust so that you can sand it better. There are small sand blasters for home owners but they are too wimpy for jobs like this. Priming is critical for a good paint job - use good quality paint sprayed in multiple thin coats, not one thick coat. Follow with the best paint you can afford if you want to keep the trailer, otherwise use Sears Weatherbeater (no, not really). Even with a good paint job, you should keep after it so that scratches, scrapes, etc. get touched-up if you want to make your work last. You can probably tell I have done this myself which is why I own a really nice aluminum trailer now. I have used aluminized paint on a steel trailer which really made it look like aluminum but it was like an ablative bottom paint (big boats) in that it always rubbed off when you handled the trailer. I sold that one but heard later that it still looked good years later. Good luck with your project and start saving for the aluminum trailer of your dreams - if money is tight, sometimes you can get a good deal on a used one if you keep looking.