Okay, quick summary of a handful of things...
Voltage and Wattage are both ways of adjusting power - volts are a "messier" way of looking at things, watts are the end result. To spare you a science lecture, the more of either you have, the more power you drive to your coil and the more (or hotter) your vape gets, up to the point you start burning your wicks.
Adjusting power delivery can make a huge difference - don't use enough power and you won't heat your coil up enough to vaporize liquid. Too much and you risk burning your wick and getting a "dry hit." You don't want one of these.
@Zamazam is right in that Nautiluses are right at home in the 12-15 watt range, you'll want to play around a little bit once you get your device and see what works for you.
I've stopped recommending the iStick lately, I bought a 50 watt version and it's had several issues. Many others have reported other issues, ranging from the annoying (rattling buttons, poor electrical connections) to the downright dangerous (mods catching fire while charging). Most of these problems seem related to the 50, so you might be okay with a 20 or 30 - either will deliver plenty of power to make a Nautilus work nicely.
All of the iSticks have fixed, non-replaceable batteries - could be nice for the simplicity, could be a problem because you've essentially got to trash your mod once the battery dies. You might want to look into a replaceable battery mod - the cost is a little higher up front for a device, batteries, and charger instead of just a device on its own, but then you'll have the batteries to use on other devices, or if you stick with just one you can replace a $6 battery instead of a $50 mod once the cell dies. I know
@UncleRJ is a big fan of the Sigelei 30 watt box - I have a Sig 100 and have loved it so far.
