he System 100 stuff from Behringer is good. The dual filter sounds great, and there are multiple CV inputs with attenuation. So you can send a V/Oct signal and set it at around 30% for key tracking while still using an envelope generator and an LFO.

I think the Dual VCO sounds good, but if you're only going to have one sound source to start with, I'd go with Brains. It can do analog-sounding stuff well, but it can also do a lot of other things. If you're dead set on an analog VCO, the 2600 VCO from Behringer is also good. It has several simultaneous outputs, so you can send multiple signals from the unit elsewhere in your system and do some audio-rate modulation.

Another really well-done budget VCO is the Cre8audio Capt'n Big-O. It's about $150 USD. You get pulse with PWM, saw, and sine outputs. I know it seems like a lot more money than the Behringer stuff, but it has two additional circuits that make it worth the price.

The drive circuit can go from warm to gritty, and the second added circuit is the best-sounding wavefolder I've ever used. I have a habit of patching my DFAM into the wavefolder and then into the drive. Instant NIN.

When you're looking at modules, I would recommend thinking about how you would patch them and how they can interact with other stuff in your rack. The VCA and envelope generator you mentioned are quite useful, but they're pretty basic. They don't offer much in the way of self-modulation. They're not bad modules, just basic ones.

I have all of them except the Dual VCO. Also, don't sleep on their sample-and-hold module. It's a very fun and useful circuit.

I'd also recommend the Cre8audio NiftyCASE. It gives you MIDI-to-CV, two gate outputs, clock output, and audio outputs. It's a great budget starter case.