I usually only need to use a 75 watt Zoo Med spot to heat a 20L tank. Buy I have mine above the screen not on it. If it's on the screen you may only need a 50 watt. You put his basking spot (log whatever) on one side and put the light above it. The other side of the cage will be cooler this way and will allow him to thermoregulate. So I think a 100 watt may be a little much but you'll have to check it with a thermometer as I mentioned before.
Don't get the heat pad. They have no value with bearded dragons. I only use them in my nest boxes, never in a dragons regular enclosure. Do go with the UVB fluorescent. If you are going to lay it on the screen then you should get a Zoo Med 5.0. The length of the tank would be best but since there is no 30" your probably going to just have to get an 18" and put it on the hot side of his cage. Also be sure you dust the crickets with Calcium w/D3 (No Phosphorus or Vitamin A) every meal except for one a week that you dust with a multivitamin. I feel that RepCal is the best products for this.
Below is a link for greens and vegs. I'd start him on a mix of romaine and mustard greens torn into small pieces no larger than his head. Salad should be offered every day regardless of whether he eats it or not. I put the salad in about an hour after the first cricket feeding and leave it in all day until the last live feeding.
http://www.beautifuldragons.503xtreme.com/Nutrition.html
Oh and congrats on your new dragon!