I have done a lot of research and have a bioactive tank for my crested gecko. For a humid environment with little to no sand in the soil mix, a drainage layer is needed. For a more arid environment it isn't necessary. If you plan on eventually adding an animal to the mix, I would build based on the temperature and humidity of the species you wish to house. I got a lot of good info from NE Herp.

My biggest worry with bioactive is trying to do it with a species that needs belly heat. There just isn't a good way to use a UTH because the substrate has to be thick enough to support the plants. Proponents of bioactive setups say that it is ok to skip the UTH and heat with CHE or RHP, but I am nervous about trying it. I would like to convert my Leopard gecko but the idea of ditching the UTH holds me back.