sparkiesford - Congratulations on the upcoming bundle of joy! It looks like you're pretty well covered with lighting options here.
In regard to just how much you need, it really depends on what type of plants you want to grow. Plants such as
anubias,
cryptocorynes,
java fern and
java moss will all do well without intense lighting, though any plant will grow more with more light. I've even found my amazon swords do well in my 40 gallon aquarium, which is running on standard lighting.
The light that comes with the tank is usually between .5 and 1 watt per gallon. If you want to grow a wider variety of plants, you'll want to be in the 2+ watts per gallon range.
You may also want to look into dosing with something like Flourish (manufactured by Seachem) to aid in growth. As I mentioned, I have amazon swords in my 40 gallon with about .5 watts per gallon and they are doing well. The only thing I add is Flourish and I tend to overfeed my fish a bit (not recommending that) which will make more "nutritents" for the plants as well.