nitrate, ammonia and those poo poo stuff will mostly remain at the bottom of the tank if the water is not strongly circulated, which also means that plants inside can help to break down a portion of it, but then still there are certain amount that are every where in the tank, so like you say, try it out
moisture of the medium...I personally think the medium will be wet, good enough for other cp that require lot of water like drosera, sarracenia and vft, nepenthes wise, those intermediate and high land plant can be given up in this project since they require more airy medium...low land variety might thrive under such condition like gracilis, mirabilis...but then the size those sp can grown into is pretty big or tall
with a tank of water underneath the nep...humidity is expected to be high, a good thing for most nep
looking at how the medium affect the water quality...perlite might release chemical not good to fauna in the water, need to do some research on that before using, sphagnum moss is very acidic, I read somewhere some times ago which stated it can lower the water to ph as low as 2 if the water quantity is very very little, but for a big pond of water, no idea what will be the overall effect...
algae wise
, once sphagnum moss is infected, hehe, almost impossible to get rid of, they might "expend their territory" to other part of the tank...
look out for lighting stuff, vft and sarracenia thrive under very strong lighting while most drosera can tolerate low lighting area but won't grow into it's best coloration...
good luck on that and keep us update with some photos when you made it