The more light the better, the difference in price between a 600 & 1000 is nominal so go for the 1000 watt.
You cant harm a plant with too much light, however you can burn it by the light being too close.
Each persons set up is different so you will need to fiqure what best works for you.
You should also get a temp. & humidity meter so you can monitor and adjust your space.
I would read up on this site and dont rush your choices.