places to look for used modules - modwiggler, here and facebook
there's also wigglehunt - which is an aggregator for shops + reverb - but misses a few I think
you may or may not need an output module... I don't need one for example (UK and don't play gigs - where a balanced output may be an advantage)
if you're going into a mixer or audio interface I would try passive attenuators 1st, before spending money on an output module!
"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