There's basically no nitrifying bacteria in the water column, so all using "old" water will do is add whatever pollutants have built up in your DT to your QT. I don't recommend it.
While running a cycled QT is best, you can run an uncycled QT, but it simply takes a bit more work on your part, as if the tank is uncycled, you must not let it cycle while in use, which means more water changes.
You don't really need a light, and in fact, most newbies should be kept a bit dimmer, as it stresses them less. However, you DO want to light the tank every so often so you can check the fish for parasites/illness.
QT's can be as basic as a 10 gal with an airline (the uncycled version) all the way up to a fully cycled/decorated tank that is kept running and "ghost fed" to maintain the cycle.
Here's a good little primer on various QT methods:
http://www.lionfishlair.com/qt/qt.shtml
HTH