Don't use live rock or any sand. The only time sand would be used is for fish such as Wrasses that would burrow in the sand when frightened or at night when they sleep. The reason is rock and sand will absorb the medication you would put into the system. Or, if you would go with hypo salinity as a treatment it would have a negative affect on the beneficial bacteria in either and be useless. Best to keep things sterile and simple.
I would use a large sponge filter for the system if you are medicating along with a powerhead depending on the fish. Just a powerhead if doing hypo. The reason I recommend having at least two nice sized powerheads for the system is it gives you control. When I QT'd my Achilles who was real sick when I brought him home I gave him a ton of flow in his QT and I attribute that to his survival since that is one of their key requirements. So depending on the fish being able to adjust the flow in the QT system will benefit them. However, the powder blue in QT recently did not like flow right away. So just a sponge filter was used.
My QT set up is the tank, heater, sponge filter, powerhead(s) when needed, shop light, and a pvc tube appropriate for the size of the fish. Anything more is complicating it. I am not sure about using the skimmer so you will have to wait for someone else to comment on that, I feel it may be best to just add that into your main system and feel its over kill for a QT. Even a sump is a bit much, I prefer to keep it simple. Although, a permanent plumbed 55 with sump and skimmer sounds like a killer QT setup and a major asset.
Make sure you have dedicated siphon, a bucket for QT use. You absolutely do not want to mix QT items with display items.