Sorry but Watts do matter to some degree.
Yes the K rating matters more, but if you're going with lower light plants, it matters less.
As I said, the light will be fine, but such a large light may be overkill in a few ways. 1: The look. 2: The inhabitants may be a bit overwhelmed, you never know 3: Depending on the type of bulb you pick, the heat.
Fr0sty, don't be confused. Ferts do different things than Co2 does. They are NOT the same.
Marimo balls will be fine in your tank. They're algae and very resilient.
If you do choose to do Co2, you'll have much less of an algae issue. But if you keep marimo balls, RCS and maybe ONE nerite snail, you could easily pull off an algaeless tank.
Floating plants will not inhibit low light plants, like Anubias Nana. I would avoid duckweed, tho x.x
Without floaters, you could certainly try some higher light plants.
This brings me to my final point: You will probably need co2 with such a strong light.
(A neat plant which will show you if you have low or high light is ludwigia repens. It turns red with high light and green with low light.)