It's a hard question, what to start with. What is a minimum viable synth that is fun enough that you won't be disappointed and sell the whole thing in frustration.
Key is versatility I guess.
I don't know Permutation but instead of that I'd recommend Pamela's New Workout. It can generate gates (logic, euclidean, random etc), CVs (also quantised, random, looped random etc), clocked modulation, simple envelopes, LFOs, random stuff on 8 channels. It's great to start out with and it remains great in a big system.
I suppose you saw/heard Cloud Terrarium online and you like how it sounds. I think there is nothing wrong with going ahead and getting it if you can afford it. I think it's important that you find your modules exciting.
Instead of the Mix3 get an output module that has a built in mixer, like 4ms Listen Four. You need to attenuate Eurorack levels to line levels, and maybe you'd like to attach headphones. Get a submixer once you have more voices. Mixing CVs is also important, but Maths will help with that too. For later check out Happy Nerding 3xMIA:
Maths is great to start out with and remains great as your system grows. It can even be used as dual analog oscillators that can also track V/Oct to some extent in the lower frequency spectrum.
I love effects, get a Happy Nerding FX Aid (maybe XL) if your budget allows. Filters are also great but you can start without one. You can even create filter-like effects with Maths.
Mantis is a good choice as a case!
Edit: almost forgot, you will need a VCA from the beginning. Without one, you will only be able to make drones. Get one with variable response curves (exponential for audio, linear for CV). Mutable Instruments Veils or Intellijel Quad VCA is generally recommended.
Also, if you have other hardware, like a drum machine, find a way to connect them to your modular. For MIDI there are many modules, like the Mutant Brain. For connecting to a DAW the Expert Sleepers ES-8 or ES-9 might be the best, but there are other options too.