I'd lower you temp to the mid 70's which will be much better for most plants.
As long as the gravel is about 2-3mm it will work just fine. You can replace it if you want, but it isn't necessary for plants. Going for something finer would help with the carpeting plants of course.
I believe that the Current Dual Satelite 20" (2x40watts)
may be exactly what you're looking for. You'll need to do some measuring to make sure that it would fit all right. This would probably get you somewhere around medium high light. It's not quite enough to be sure you'd be able to do a glosso carpet, but it would allow you to grow a wide assortment of plants. If you could get ahold of some clover or Elatine Triandra those would have a better chance of forming a nice carpet under that level of light.
If you go with the above light recommendation, you'd need to setup CO2
and remove the biowheel. Ferts would also be a necessity.
For a low light solution you could try to get ahold of a double 20" flouresent light strip (2x15). This would be a bit less than ideal for low light, but would let you grow the basic low light plants (Anubias, Crypts, Java Fern, Java Moss, etc).
For a medium low light solution you could look at the Current Single Satelite 20" (1x40watts). This would give you a slightly larger selection of plants but still wouldn't let you get into the carpeting plants.
Both of the last two solutions wouldn't require CO2
or heavy fert dosing.
Please keep in mind that you would need to replace the bulb in either of the Current fixture since Actinic light is mostly unusable by plants. The replacement of the bulb should be factored into the cost of the fixture.
One final option would be to buy the first fixture but to only run one of the bulbs. This would allow you to start out with medium low light, but then move up to more light when you are ready without having to invest in a new fixture.