That's a fine set of gear. Not sure about the longevity of the iStick 50W (only because they've been available for, like, a cup of coffee), but the subtank will allow you to use either premade or self-built coils, so maximum flexibility there. For that I'd choose it over the Atlantis.
An alternative for the iStick might be the IPV Mini V2 (70W). Probably a "pick 'em" situation.
The Mutation is a good atty. Just note that the fashion of RDA's is very fast moving these days and you're going to see that those on the cutting edge will move on - just don't get discouraged when that inevitably happens.
28 gauge is the perfect general purpose wire - very versatile and easy to work with. You might find you want to try bigger and smaller gauges at some point, but having 28g around never hurts.
I'd recommend Koh Gen Do cotton over shiseido (I've tried a lot of both) - it's more raw and looks not to have been bleached (the shiseido I had was whiter and more homogenous). Opinions will vary on this.
As to whether or not you should get it all together, well, you need most of it at once to set up a dripping config. The subtank is the only "not like the others" item, but it gives you a tank - very useful for out and about. I'd say go for it if the budget allows.
