I don't think the undergravel filter is something your going to be happy with in the end. I actually didn't even think they sold them anymore. The sand bed can be used to grow beneficial bacteria that will in turn aid in the biological filtration. This is why a lot of people believe in the use of the DSB. The LR you are using in your tank will be able to cover this though, and will actually be the source of your filtration, so you could get away with just a couple inches of sand. By using a reverse undergravel filter you will be constantly disrupting the sand, so you should have a nice clean sand bed, but that's about it. IMO I would do without, it will probably cause more problems than its worth in the end. For just a standard FOWLR tank you actually don't need much more than your LR for filtration. Make sure you add a couple powerheads for flow. I personally wouldn't run a system without a skimmer either, but thats debateable.
Hope this helps a bit
