the reason I would buy an effect over quadrax is that you will then have 2 modulation sources (Maths and Quadrax) - whereas I would suggest reasonably early on adding something that is modulatable and will process the audio from your other synths - you may find you want something like quadrax next!
whilst I think Lugia's recommendations are always excellent, I would suggest taking them, and mine (and other people's, for that matter) with a little salt - they are much more about functionality, than specific modules, even when specific modules are suggested/recommended - except Maths and Veils!!! hahahaha
Part of modular is about having your own custom synthesizer - mine for example has 11 sound sources, one of which is polyphonic, and synthesizes both audio and video - it is quite large, I suppose, @ nearly 1500hp - NB the most common manufacturers in my modular are Doepfer, LZX (video) and Mutable Instruments (although some of both the LZX and mutable are DIY)
Even if you do buy all of Lugia's suggested modules, there's nothing wrong with swapping something out for something else, either if you don't get along with the specific module or can't find it due to availability issues and desperately need a XYZ module immediately
"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