wouldnt bother using uv with whistlers (being nocturnal), wouldnt hurt having a low uv output bulb for bell frogs.
flies ok but not the best. try get some locusts, crickets, moths, slaters etc into the diet.
only need a little bit of water (1/3 or less). keep clean. have seen many set ups with way too much water, simply not needed and just reduces areas for the frogs to forage.
substrate can be much the same as fish tanks. i use daltons propagating sand a lot (with a few river stones to bulk it out/create depth/land areas etc). and or river sand. i do try cover the substrate in javamoss/xmas moss to get a lawn effect also lessening the chance of the frogs eating the substrate.