big mistakes to avoid?
buying modular synthesizers without enough research to know what you are buying...
Do I need outputs and/or mixer or some other modules can do that?
output module: you may or may not need an output module... depends what you are connecting to, most modern mixers and audio interfaces should be able to handle modular levels
mixer is probably too small for this size of case
you probably need more mixers
see my signature for a vague hint at how to build a modular synthesizer with the most variety for the least amount of cash
"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