Minimum preparation for baking a whole fish
You will want to scale the fish, definitely. If you don't the result will be absolutely awful as the scales will come off in cooking and get all over the place. They won't stay attached to the skin for easy removal. 

Other than gutting and scaling (and a thorough wash) preparation before cooking depends on the effect you want on the table, and how much you want to de-bone it. I'm not a fan of fish heads so I remove them before baking, but you may want to leave it on if you like that look. I usually remove all the fins but the tail as well as I find that it's easier to do before cooking than after as you can get lots of little bones from them if cooked. 

Other than that you can marinade them, or salt the insides for 20 minutes to firm up the fish, but that's totally optional.