you basically sand it - start off rough and finish with a fine wet and dry. Use a flat square block that doesn't flex so that you only grade the raised surfaces ]
you should be at it a while as powder coats fairly solid
possibly a flat power sander, i wouldn't use an angle grinder or drill with a pad as their not accurate and you only want to remove the paint and not them metal
yeh, if you finish up with a 1500 wet and dry and then use some any kind of metal/bodywork polish afterwards - aluminium is soft and relatively easy to polish