Your latest build is enough to get started. Save the empty space for future expansions once you know what you'd like to do. The two Quadratts should be enough for some basic mixing of what you've got. Plus you can mix with the quad VCA as well. However, you're limited to mono. As far as mixers, Happy Nerdings Pan mixer is good, Roland's 531 Mixer, they should be available in Europe. I think there are some performance mixers you might like and Lugia mentioned another make that has aux sends on it.
You have two dedicated ADSRs. The O&C module can generate up to four more as well as CV using the Sequins mode. You'll find envelopes great for controlling volume. But they are great for modulating other attributes like your filter cut-off frequency, Q, or even modulating the pitch of a VCO... like when making a kick drum for example.
Take a deep breath and pull the trigger on this. Get reading to learn a few things. :)