I'll just expand a bit on the suggestions from SinCrisis (with which I concur).
Undergravel filters can work, there are too many planted tanks using them to say otherwise, but they are not the filter of choice for a planted tank. One problem is that plant roots will clog the plate openings. Another is that the water is drawn down through the substrate faster than it should be for optimum plant growth. In the substrate there is a very complex cycle going on, between bacteria (here we are talking bacteria different from the normal nitrifying bacteria), plant roots, and nutrients brought down in the water. There is a natural flow of water through the substrate in the aquarium just as what occurs in nature. It is always best to let nature do its thing without interference; it usually means more success, with less to go wrong.
As you may be switching substrates anyway, removing the UG filter plate would be recommended. As for the substrate, a finer substrate works better both from the aspect of anchoring the plant roots and be assisting the afore-mentioned complex activity. In my 20+ years I have used just about every substrate material there is, except for soil [which is a whole different issue], and I can say with certainty that plants do best in a small-grain fine gravel or sand.
For some background on these and other issues, like the light question, have a read of my article "A Basic Approach to the Natural Planted Aquarium" at the head of this section of the forum. There are 4 parts (too long for one post
) covering the basics of a natural or low-tech setup.