Most external gear can cope with euro level - just turn the mixer down - if you have it at full volume it probably will clip - but start without a output module - if it clips then try a pair of passsive attenuators - and if they still clip or you NEED balanced outputs then buy an output module
the argument about mults was mostly about the fact that you don't need buffered ones for anything other than v/oct - but they were a bit excessive - passive mults are cheaper and don't take power headers - trying to stop you spending money on stuff you definitely don't need
the expander for zadar is very useful - more modulation inputs and you can mute the envelopes both of which are pretty handy
as for Maths - it's often left out by people who are willfully defiant due to the fact that it's popular - and as others have said - experience of fixed-architecture synths is really not as applicable to modular synthesis as it would seem - it's great for patching basic east coast style mono-synths and that's about it
personally I'd want more vcas and a matrix mixer
if you want to see my rack - look for this one : All My Eurorack includes DIY backlog and pre-orders, etc if it's upside down it's not there yet!
"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