They make fine pots for lilies and marginals, and they last almost forever. They are just a little too small for lotus. With lotus, bigger is better, they like lots of room. My friend Anna grows her lotus' in them and they do marvelously. Her barrels are around 28 inches deep with 8-10 inches of water above the pot.
Dirt is pretty simple.
As long as no pesticides, fertilizers, or things like that have been used on it, dirt is dirt. I grew lilies in pots and goldfish in rain barrels for 2 years before deciding to build a pond. Most of the dirt for the major repotting this fall came from digging the pond this summer. All my lilies enjoyed the new dirt and the extra room.