I would buy the case, Maths, Disting, Veils, one of the vcos (BIA or Plaits) and one of the effects units and start with that - I'd also add MI Links and Kinks (as a superb utility starter set - you need more utilities) - and learn those modules well (including a few from Disting)
that may leave you with some cash in your pocket for future purchases once you actually have played with a modular, and then go slow - it's not a race - when you are ready for another module - add one or 2 modules at a time*, learn how to integrate them with your existing modules and repeat
*some modules benefit from extra utilities to get the most out of them - clouds needs attenuators, for instance, so sometimes a couple of modules are a better purchase than just one
"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