Copper, brass or bronze, depending on the hardness desired, but most often brass. If the rusted metal is the bright steel (which relies on its polished surface to resist rust) of the PSU's case, or of the RFI shielding that I could not see in your pic., follow Dana and Bunsen. A brass-wire brush will scratch the bright surface least, but use it only to remove loose/powdery/erupting rust. Don't try to get back to bright metal—leave some rust colour—and then stabilize the surface with a proprietary liquid (such as Kill-Rust, Neutra Rust, End-Rust) containing chromium salts, phosphoric acid and poly-(vinyl acetate), applied by rolling on with a cotton-bud (cotton tip). Some light rust must remain to take part in the chemical reaction, and the PVA provides a protective coat over the result. You may not like the resulting deep blue/black colour, but further rust should not develop. You could also just brush off the rust and rely on keeping the inside of the computer dry.
Steel that has been annealed, plated or galvanized with zinc (dull to bright grey in colour) should not be treated with the same solution. In this case, brush, water-wash, dry, and spray-lacquer the affected areas. The 'sacrificial' zinc layer has done its job; the resulting salts have been washed off, and the zinc cannot be replaced easily. Or just make sure that the metal stays dry.
de