I've read its good to put artic blue lights on 30-60mins before and after the white light to replicate dawn and dusk, does anybody do this? Does it benefit the coral?
Lots of people do, and im sure lots don't. I like the look of the dusk/dawn thing.
Also hows a good idea to form a reef stand to sit live rocks on? what can I make it out of?
Yes, its good to keep as much of the rock of the bottom as you can to allow water to flow thru.
You can use, cut pieces of pipe, bar mesh stools, egg crate (light difusers) or and undergravel filter plate. You could also drill holes and insert acrylic rods what ever you like really.
Is an undergravel filter a good idea?
NO
I've got a 30L tank with heater and basic tropical filter system, could I use this as a hospital/qaurrantine tank??
Sure
My filter is a Jebo 819, its got several different media, what order should they go in?
Don't bother even using it