Another option is to buy a prebuilt system that comes with the sound, modulation and utility support modules as well as the case/power for learning purposes. Some good ones are:
Doepfer A100 Basic System- this is how I started my modular journey and comes with all key basics like VCA, mixer, etc.
Erica Synths Black System- lots of great modules plus CV tools and mixer
ALM Busy Circuits System Coupe- this one just came out and looks like a fun modular jam station with sampler and goodies.
Make Noise Shared System Plus- west coast off the beat modular with tons of great all star modules like Maths and Rene
Endorphin.es Shuttle System- another funky west coast modular system.
I do recommend the free VCV Rack software, I still use it for trying route options out.