Hello ziopin and a warm welcome to the forums. If the galvanising on the tub is sound I wouldn`t coat the inside with anything, just scrub it out with hot water. Don`t use any soaps or detergents to clean it. You could paint the outside for decoration. A mixture of gravel and sand would be fine. As for planting chose a pygmy waterlily (they are not cheap) for flower and shade. One of the best plants for oxygenating is Hornwort, it doesn`t make roots and attracts waste matter to it, you could also add a little duck weed. don`t over stock with plants at first. Fish and pond life don`t mix well, pond life is food for them
I would go for one or the other. A ramp can be built from rocks at one end or a branch of wood weighted at one end and leaned against the side if you go for pond life. Keep out of sun as it can get quite warm being metal,and out of strong icy winds in the winter. In the winter you could try bubble wrap round the outside to prevent freezing or place hardy pot plants round to give shelter. Good luck with your project,,,Bob