The Atlantis is a good starting point if you're building a small system first and then adding to it little by little.
But if you're going to dive in fully, then it starts to lose its luster. The plus side is that the patch points are pretty much all there to break the unit out into its individual components (oscillator, filter, etc). The downside is that it's all or nothing in terms of what you get. You may do better getting individual components that will offer you more flexibility and features.
Mixers. You have a Quadratt. It's functional as a small mixer, attenuverters, etc. But you really are going to need at least two small mixers of the Quadratt nature (not necessarily the Quadratt) and a main mixer if you plan on having more than one voice or sound coming from your rack.
There are better units out there, but for the PRICE and SIZE, I'm very happy with the Blue Lantern Stereo Sir Mix-A-Lot. Gain, panning, mutes, plus two aux sends on every channel... plus a pair of stereo returns. You can do better on features... but I don't think so on price.
Also, you should have empty space in your rack. No one learns modular without getting their hands dirty. The more you learn, the more your experiences CHANGE your end goals and desires from your system. If you buy everything all at once and you have no space left over, you're committed unless you start selling stuff off... while you're still trying to learn to use what you've got. Always plan for a "whoops, I should add a..." and have the space for a couple of reasonably sized modules at least.