Duckweed will take over a tank in no time given the ideal conditions, so you'll have to be on top of clearing out spaces on a routine basis.
1. I've used gravel and sand before and would personally go for something like Flourish, if you can find some/you like the color/it's not too expensive for you. Plants will stay in gravel better than in sand, but will be easily disturbed when you vacuum. It's pretty hard to keep plants in sand without weighing them down (I made some posts on plantedtank.net about how to keep them in the sand/fine gravel eassier; I can find it if you are interested).
2. Many low-light plants will do well in your tank with no/little nutrient addition. With that being said, your plants will look healthier if you dose with Flourish Comprehensive (as noted by the poster before me). Some good low-light plants are: mosses (christmas, java, fissidens, etc.), ferns (java fern: you will have to trim them since they can get kinda long), anubias (nana), some cultivars of ludwigia. I don't know how much height or floorspace your tank has, so look them up and see what you like.
3. That bulb should be fine if you plan on keeping low-light plants like mosses or ferns. If you want to get something that needs more light (like ludwigia red), then you'll need to upgrade the bulb to have the plant show it's nice colors. Something along the lines of 6,500 - 6,700K is a safe bet. I have also had good experience with full-spectrum bulbs since they put out all bands of visible light. (http://www.walmart.com/ip/GE-3-Way-Reveal-Bulb/16880832
) I don't know the max wattage allowed for your fixture, but they make bulbs in a bunch of ranges: from 40, 60, 100, 150, etc.