a good starting point would be to get all doepfer modules (vco, vca, vcf, lfo etc) - they are reasonably priced and pretty good
the dreadbox chromatic modules appear to be quite decent too - but pretty much any brand within your price range is a good idea
for a vca I would suggest a quad cascading vca - either veils or the intellijel qaud vca would be my top picks for a starter case
otherwise maybe an idea of budget would be a good idea - i would go for simpler stuff to start with and learn how to patch before getting more (complex) modules (and get them slowly, if you can)
and don't forget to spend some time working out what different types of module do - especially ones that come under the banner 'utilities' - they are relatively inexpensive and are the dull polish that make the shiny modules actually shine
"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