just to clarify, there are strong weatherproof stains that (ime) suffice for simple projects like this...I have personally used Behr Weatherproofing Deck Stain (rated for blizzards and snowstorms) and it is going ~1yr strong without any signs of damage. However, it still isn't a "permanent" fix and a sealer is a better long-term solution.
What light that gives Vit D? I'm assuming you mean a UVB light? I'd also recommend a 5.0, and remember that those bulbs have to be changed every 6 months (they will still give off LIGHT, so you can NOT visually tell when it is time to change them...but they will not be giving off much UVB after 6 months)
I generally agree with Texas Panther Man, although the 6500K isn't necessary for the chams it does aid in plant growth nicely.