I can't answer your question about the Wisteria, but as to changing your tank over to sand, there shouldn't be any issues. As long as you keep the same filter, which has bacteria in it, that's all that matters. Obviously the substrate (gravel) has bacteria in it too, but you're not going to lose what's in the filter and that's the most important part. So I don't see why there should be anything for you to worry about in regards to changing to sand. Personally, I like anything that resembles nature, so I myself would not go for the black, but having said that, some tanks look really nice with a black or darkened substrate.
And you do have to be careful, there's alot of plants that are sold as "aquatic" when they are really only marginal plants which prefer to have their roots in water, but the plant out of the water. They look nice, so you get them and they seem okay, but then they die shortly after putting them in and end up making a mess and you've spent your money on nothing!
