For the most durable results, I’d recommend sandblasting and powder coating—of course this is not cost effective for most. If you decide to paint, sandblasting will provide superior surface prep and paint adhesion over etching products. For painting bumpers, sliders, skids, etc., I’ve used everything from high quality DuPont auto paints to rattle can RustOleum. I’ve settled on Rust-Oleum Professional High Performance Protective Enamel in flat black; the durability, finish and color retention were the best PERIOD. It’s sold by the quart or gallon (never buy black again size) at most hardware stores. If you choose to prime, Rust-Oleum makes a like professional primer, have it tinted black or dark grey so that scratches aren’t overly visible. If you paint the floor of the interior, the contents will wear the paint leaving black powder residue on your belongings. You could always line the floor interior with cheese cloth or rubber matting.
As for using diamond plate on vertical surfaces: it’s all about aesthetics. It hides fabrication imperfections and dents better than standard sheet. I used diamond plate when covering my fenders & rocker panels for a few reasons. 1) I used a step rolling method to bend the radiuses and the break lines are fall less visible. 2) Diamond deflects small objects that strike its surface a bit better than standard sheet given the dual surface levels. 3) As mentioned by ntqsd, it handles scrub brush well. 4) It looks good.