You're on the right track.
First, you're buying a case that's larger than your immediate needs. You have your future expansion in order.
The Squarp will get you going for sequencing. I'd also check out the following: Ornamets & Crime (micro) and Temps Utile (micro). Between the two you have quantizers, Euclidean gates, gate sequencers, CV + gate/ADSR sequencers, etc. You may also want a simple hardware sequencer (knobs and/or faders) like the Befaco Muxlicer. I'd also check out the Mimitec Digitalis by Noise Engineering for a 4x16 step CV sequencer (useful for modulating filters).
I think the Squarp can do LFOs if I'm not mistaken. LFOs are great for evolving sounds, especially filters. Look into getting at least two ADSRs and at least four VCAs.
Filters. Filters are all about individual taste. Don't throw in the cheapest thing and call it a day. I see a lot of builds with a Doepfer Wasp filter chucked into the rack without any consideration for the sound. A good multi-mode filter is where I would start... like the Morgasmatron by Intellijel. You get two multi-mode filters. You'll probably want to add others and other types later... maybe even a Wasp :)
Next up, switches/mutes. If you're going to be performing live, being able to mute CV, gates, and audio is very handy. DivKid just released a quad mute that also has momentary functionality. Joranalogue has the Switch 4 that can do the same but also includes a 4:1 rotary switch.
Finally, effects. If you're doing ambient kinds of sound you'll definitely want reverb and possibly something granular like Clouds or the Intellijel Rainmaker. There are lots of options.
Finally, mixers and output modules. Intellijel makes the Quadratt that fits in the 1U row. The 7U case also supports line level outputs if you buy the Intellijel output module. As far as mixers. I went with with a Blue Lantern Stereo Sir Mix A Lot. It was under $250 and has two FX sends. There are plenty of options out there for mixers as well. It depends on your needs.
If you're West Coasting your sound, check out Noise Engineering's modules. It doesn't hurt to have a few analog oscillators as well. But for the time being, a Braids or Plaits module makes a great generic sound source to add to your 0-Coast.
Odds and Ends: sample & hold, slew limiting, noise (Intellijel also has a 1U module that does this)... oh and a logic module... it's getting expensive pretty quickly...
But the bottom line would be to buy one or two modules at a time and get a really good feel for them and experiment plugging them into the 0-Coast, especially modulating things you normally wouldn't.