Maths is a good choice if you are interested in learning modular synthesis - see the 'maths illustrated supplement' - as opposed to synthesis with modules, if you see what I mean! delta v is also a good choice, but doesn't have the learning resources...
I'd go for veils over the dual vca - you can never have too many vcas!!!
Rings is not particularly similar to plaits - plaits has a couple of Rings-ish settings, but has a lot more besides (and vice versa)
otherwise @ronin1973 and @plragde have you covered
for mults - if you have 2 sound sources and 2 v/oct sequencing channels - then buffering is superfluous - as it is only really needed for pitch, otherwise passive is fine & in a tiny case such as this, stackcables or headphone splitters are a better option as they won't take space
"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