please mark your rack as public and post the url - fuzzy jpgs aren't that helpful to us helping you - no click through or mouse over - and with 11k+ modules available, no one knows all of them!!
but for starters:
sq-1: probably not the best sequencer for sequencing chords... only 1 note at a time... so no ability to go from a major chord to a minor chord, for example... don't just buy modules/things because they are cheap buy the modules you really want/need after extensive research and thinking about how you will actually use them in practice, otherwise you'll be swapping modules in and out until you do find the ones you want/need - which is a false economy - which leads me to...
oscillators: whilst these may seem like a great idea, I think you'll have issues with them... they are tiny and use trimmers and as such are probably not going to be great for precise tuning & having them next to each other will make them almost impossible to adjust when patched - so don't rack them next to each other! - remember that in eurorack 1hp is just 1/5" (5.08mm)...
a lot of people are shocked by how small modules are once they've actually seen them! also pay attention to module and case depth... not all modules fit in all cases - 2hp modules can be quite deep - as the pcbs are perpendicular to the panels - and can be quite fragile
whilst in general doepfer make some fantastic modules, especially their utilities and filters - their midi modules are best described as not great - if you need a midi module I'd go for something else, probably with more channels - if you want chords you'll probably want at least 4 channels - mutant brain might be a better option - & it has extra gate/trigger outputs so you can use midi clock for example, if you wish - but I'd seriously think about how you're going to sequence your modular - pick either midi or cv and then get a module that actually fulfills your needs (I don't think either of the options you've picked do!)
newbie question: if I don't plug in an external sequencer, do I need a clock? I worry about the sequencer being quite constraining on what I can do with this
it depends - do you want to have modulation etc that's in sync with the notes that are being played? the sq-1 is quite a basic sequencer... especially if you want to play chords! find one that's more advanced - preferably with enough channels to sequence all your voices independantly... I like the erica black sequencer... it has 4 channels and each channel has quantized pitch/gates/modulation, has an internal clock, each channel can run at a different division/multiplication, and a clock out for sending to other modules!
newbie question #2: do I miss some critical part of a functional system? A mixer, maybe? A second VCA? Also, not sure about how these 3 oscs all feed into the ADSR? (hence the mult. I don't know if that's how it's done.)
mixers are incredibly useful, you'll likely want a few of them - not just for audio, but also for combining modulation... same with vcas - useful for both audio and cv - again you'll probably want at least a few - I'd go for a quad cascading vca - such as a veils clone...
vco outs go to vca inputs - envelopes (ie an adsr) go to the cv inputs of the vca so they can open and close it - sounds like some further reading on the basics of modular synthesis is a good idea - the stickies at the top of the 1u & 3u subforum on modwiggler are a great place to start!!!
The oscs are all sines. I picked these because they're apparently quite pure, they sound great in the videos and are not too expensive, but they might not be ideal in terms of ergonomics/fine tuning and/or functionality?
if you replace might with WILL in the sentance above you will be correct!
buy vcos that are bigger, have proper knobs on them and not trimmers (especially for pitch - tuning!!) and preferably with more functionality - multiple waveforms are a good idea (options/verstiltiy), as are things like pwm... if you seriously want only very pure sines, you might find you are better off using resonant filters as sound sources - do more research!!!
there are plenty of good inexpensive(ish) vcos about... probably a bit more expensive than those 2hp ones, but not necessarily that much more! but you may end up wanting a simple mixer for each - so you can combine waveforms...
I've been thinking of replacing them with the Ensemble Osc from 4ms but I'm worried about it being a bit too ready to use/prescriptive. I like the idea of something more basic. Your thoughts will be apreciated.
ah the debate of 'modular synthesis' vs 'synthesis with modules'.... both are valid, as are all points in between... but to start I'd recommend more basic modules - making up a single voice and learning how to use that inside and out before adding more voices... at which point a module like the ensemble oscillator, might be a good option - make sure you understand how to sequence it properly - iirc you really want more than 1 sequencer channel - either to sequence root and chord type, or to send it multiple pitches to play at once - also I'm not convinced that it will do full polyphony - paraphony yes, but polyphony??? again do your research!
Chose this ring mod that apparently sounds quite clear. It might go to make room for something else. Any suggestion?
why a ring mod? doesn't fit with your pure/clean aesthetic... maybe a multi-function effect module would be better... fx aid pro (so you can see what you are adoing) would be my pick!
I'm very much into phasing tones, combination tones, Shepards. Any idea/tip about this?
phasing - as in the effect? get a phaser - the new doepfer one looks good
combining tones - mixer &/or cascading vca
shepard tones - I'm sure you can patch it but there's at least one module that has this built in expert sleepers general CV - which is a great and undervalued module... despite it's tiny screen!
Also, any tips about resonating filters? A second filter? Something else?
if you want polyphony rather than paraphony - you'll want a filter per oscillator (& possibly the same filter for each) - spend some time fiinding the one(s) you like... even if you only want paraphony, multiple filters will give you more choices/versatility, but they can come later...
I'd also recommend taking a look at my signature file & spending a fair amount of time thinking about what it says - the formula is a hint at (loosely) getting the most versatility from a modular synthesizer for the least expense...
I hope this has all been helpful...
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities