The OP is building a simple monophonic synthesizer. Honestly, you'd be happier buying a TB303 clone or a Neutron. They are much more sophisticated than the OP build and a lot cheaper.
An oscillator, a filter, a couple of EGs and VCAs... there's no need to get into modular for that. Where modular shines IS the modulation. Pick-out and design your modulation FIRST then worry about oscillators and filters. You'll have a much better time even with the most basic of oscillators with some great modulation, than you will have with the most complex of oscillators that only have two EGs and an LFO to play with.
The most overlooked functionality are EFFECTS. Most synth patches sound pretty dull and dry without effects. But they seem to be an afterthought when people are first putting their kit together. Do NOT skimp on the effects. They are 1/2 of your sound.
Don't be afraid to be wrong. You'll make a lot of them. You'll have lots of modules you buy today that you'll regret (not deeply) later. Keep them around until you know you'll never touch them again. You'll also find the modules you curse today might be that special dash of seasoning later on.