What Jim says! I started out almost year ago and, with his advice, I got myself an Intellijel 7U 104hp case. He actually advised a Mantis, which is much cheaper but has no 1U row. I built a second case (DIY) in the meantime without 1U and I have to admit I don't miss it. I'm using only half of it on the 7U anyway. In any case (haha, get it? case!) a bigger case is very advisable because as soon as you're adding a few modules, you'll curse the fact that you have to buy a bigger one. Might as well get that bigger one right now.
Plaits and Beatstep Pro is what I also started with, and a CEM3340 based analog oscillator. Plaits is a bit of a taster menu. Lots of modes to try out, e.g. a bit of FM, a bit of wavetable, a bit of physical modeling, and so on. When you want to go further, you can get a specific module in that area later on.
Modular playlist on SoundCloud