filtration via filters is generally not used due to the need for very clean tanks (eg very little nitrates too), and filters are nitrate "factories" that produce and store it. In UG filtration it is very hard to clean this out because it is your substrate that is the media. LR basically is all you need and a decent flowrate.
If you plan on keeping corals then air bubbles can get caught in the coral and cause a "disease/reaction". I can't remember what, but I do remember reading about it.
On the torch and toadstool matter: get yourself into a local club/society or coral frag type forum where you'll probs find people selling/swapping/giving little pieces of coral cheaply.