Thank’s for the advice, let’s go for the mantis.
At the start I wanted to creat a finished case, but yes your are right.
Let’s start with a bigger case, and grow slowly.

Some advice for some modules to start?

-- supernatur

a sound source, a sound modifier, a modulation source, a way to play, a way to listen... maybe a few utility modules to help join everything together nicely...

a sound source - plaits is a common recommendation - there may be some available - otherwise any basic analog vco will be fine - doepfer or the like

a sound modifier - a low pass filter is a good place to start... I particularly like the SEM - or possibly a delay or multi-fx if you get plaits (as it has a built in filter) - so maybe an fx aid (I'd go pro though for this tbh! - so you can see what program you are on easily)

a modulation source - personally I'd get a maths - and download the 'maths illustrated supplement' work through that and try to understand what it's doing, how it's doing it and why, but a lot of people find that overwhelming (complex is just lots of simple!!!) - otherwise something like batumi is good - 4 lfos at once, different waveforms etc etc

a way to play - a sequencer, a midi->cv module etc etc etc

if a sequencer - get the one you like the workflow of best - don't be afraid to get a multi-channel ne especially if you intend to expand to multiple voices in the future - as then you will only need to learn one sequencer and avoid another learning curve in the future - I like the Erica Black Sequencer, but this is very personal... do your research - you will get lots of recommendations ie everyone has a different favourite...

a way to listen - really depends on what else you have and whether you think you need stereo from day 1 - plaits is dual mono, SEM filter is mono, fx aid is stereo... I'd just get a decent quad cascading vca (veils if you can find one) and use that as a mono or stereo output... at least for now...

"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia

Utility modules are the dull polish that makes the shiny modules actually shine!!!

sound sources < sound modifiers < modulation sources < utilities