Designing your own system is awesome. But that might be a bit much for someone who doesn't have any hands on experience in modular. No one starts out knowing what they are doing... only assuming that they know.

Make Noise and Roland have some nice preconfigured systems.

The 500 series from Roland is pretty much a classic modular system in Eurorack. You can't go wrong there.
Make Noise has a few contemporary preconfigured systems that incorporate digital modules.

Behringer just came out with their own preconfigured systems as well. I don't think you can go wrong there either. Behringer is notorious, though.

-- Ronin1973

Thank for your advice!
Maybe I will start with those Doepfer modules. After adding a filter,vca, attenuverter, would it be a usable synth voice?
I would like to learn about the signal flow, and then the modulation, effects,... Where could I read more about it? Is there any threads that will help with basic knowledge?