Lugia is 100% correct and I even found a Doepfer 6U case way too small eventually and 12u-14u is perfect for starting with room to expand for modular setups and still include VCAs, attenuators, logic and mixer modules. You could get a divkid 0chd lfo for modulation and a quad vca that would give you plenty of mileage. Add in a Kinks and Links for logic and mults and you are off to the races. If you look at prebuilt modular systems from popular modular companies like Make Noise, Doepfer and Erica Synths, they will have a single oscillator and filter and the rest are support modules like VCAs and so forth. Have you considered a smaller prebuilt system like the ALM Busy Circuits Super coupe? That has all the building blocks and sounds amazing way to learn modular and make cool beats.