@mangoest that is correct - to simplify things, we’ve created the Eurorack modules kit, which includes all required modules and splits them up to “their” respective modular PCBs - it also includes the frontpanels for a eurorack modular system.
But it is of course also possible to go with the older DOUTx4 as well! You can also mix and match, i.e. buy some of the PCBs that are normally included in the eurorack modules kit, but not buy the frontpanels and build your own, if you have a 4U system. This might also make sense, if you cannot easily get a DOUTx4 PCB or if you want nice output signalling (biploar LEDs per output).
When you create your modules, I’d highly recommend to also build a clocks module, too - SEQ v4+ has a great implementation of customizable eurorack clock outputs (you can configure eight clock outputs and clock dividers that rlate to your current BPM) - these are super nice in conjunction with any modules that are “pingable” or can process clock - i love to use them in conjunction with a simple diode low-pass gate (LPG) module.
Best regards and have a good evening!