Before you buy anything, my advice is to download a free copy of the VCVRack software and play with it. Also the book Patch and Tweak is awesome for beginners. For first modules, get a larger case than you think to have room to grow. I recommend something like the Intellijel 7u case that has room for 1u modules which are fantastic. Focus on core utilities: LFOs, matrix mixer, a filter, a VCO and so forth. Like you can get an Intellijel Quadratt, and some mults plus attenuator to have features as modular places greater need on these utility modules than traditional fixed hardware synths.