....see if I can get this before flybynight....
The general rule is to start at the lowest... I believe sirflybynight said 6w was the lowest, so start there. UP the wattage until you get the vape that I like, but I personally don't recommend going above the 4.5 volts... II switched from watts to volts because the istick will do all the calculations automatically... [Doh, he posted his reply while I was typing]. As you change the wattage, the voltage will change as well. Depending on what coil you have in there, the watt number might be different from one coil to the next. So, just up it until you find a setting you like.
Changing coils, depends on your juice. Some juices will "gunk up" the coils faster. Which means you'll have to replace them more often. That depends on the voltage/wattage settings as well. There's no real clear answer for this, but you'll notice it needs changing when it just doesn't taste like it should. You can attempt to de-gunk it by taking it out and soaking it in vodka.. that might loosen up some of the gunk. just make sure you dry it completely.
The nautilus tank will give you instructions on how to change the coils. It's really simple though.. you just unscrew the tank fromt he coil.. then unscrew the coil. It's about as difficult as changing a lightbulb. If you can clean the coils, then by all means, you can reuse them. When they get too far gone though, you do simply throw them away.
I learned to rebuild early, so I would rebuild my own.. however, I am a natural tinkerer, and have been known to "re purpouse" things around the house.