Hello and welcome to Eurorack.
Your first step is going to be a semi-modular synth. Cool. They come in two basic flavors... even though they aren't technically the same. The first is a true semi-modular synth. It's stand-alone in its own case with its own power supply. The other notion is a "synth-voice" which is a Eurorack module that's a complete synthesizer. You'll find both out there so take a look around.
If you buy a true semi-modular that can be mounted in a Eurorack case... don't mount it in the case! At least if you're starting out. There's no reason to do so unless you're looking to be mobile often. Use the saved rack space to build your Eurorack module collection.
The Moog Mother is a good choice. There's also the Behringer Neutron, Behringer Crave, Minibrute (no keyboard), and others. I'd consider your budget and your skill level. If you're new to synths or analog then I'd start out small and cheap and THEN start adding RackBrute or another case later. Don't try and buy everything all at once. You'll probably spend a lot on stuff you either won't use or won't care to use. The Moog's big advantage (other than that Moog-sound) is that it has a built in sequencer.
For your rack... consider effects like reverb, delay, ring modulation etc. You may also want modules like logic (AND,OR, etc.), attenuverters, small DC coupled mixers, filters that are unlike what you have in your semi-modular, low pass gates, or even a complex oscillator.