Hmm, it's difficult to make an oscillator choice for you. It's really personal. I think you should listen to as much material as is available for the Dixie, Sto and Buchla and choose based on that. I feel like the Sto is the 'weakest' choice here, it has the least amount of options and the least amount of knobs to turn, for the same price as the other two. But if you really like the sound of the Sto, then go for it.
The Wasp is cool and has a unique sound. It's a good choice. If stereo image is important to you, a stereo filter is a good choice. The modules you have selected now are not very focused on stereo. But if that's the way you want to move in the future it can help to go stereo on the filter. Even with a mono filter you can stack the oscillators in the mixer or the Quad VCA to filter more than one sound source at the same time.
For me, the module (it's a filter) that made me go 'wow, now I want a modular system' is the Schlappi Engineering Angle Grinder. In default settings it's a sweet sounding filter with low pass, high pass, band pass and notch, but once you start turning knobs, it will rip your ears off. For me, that is the sound I am looking for. I'm not sure if it's the module for you, but give it a listen on YouTube.