Replace the bulb first of all, the one linked is a good bulb to buy. I haven't had that particular tank so I can't comment on how good everything is.
I would go with sand over gravel as it is really good for planting and promotes good root growth. I would just buy a 50lb bag of play sand at Home Depot or Lowe's, that would only run you like $5 and you can keep it for other tanks or do a project with it. It's found in the isle with the concrete mix and all that.