How would you fill a 2x20 case?

This is not so easy to answer.

The genres / styles you mention are very different :-)

For trancey stuf you ned rythm and baslines + leads so Topograf + Kick + Drumkit for the Rythm. at least one full synth voice (VCO + VCA + VCF + ENV) probably more than one.

For Steve Roach stuff i would add the Solina and the MULTIFX + loads more (but it is not easy to play chords with simple modular stuff)

You always need a few LFO's for clockimg and modulation.

For "Space" you would probably need a sequencer and two full synth voices

You definitely need at least one MIXER 4-4

Noise is good to have, so is sample-hold.

It would probably be wise to start out using grid.aemodular.com/ to try out some different layouts before comitting.

Maybe also a good idea to buy the most basic stuff first, get to know it, and then add more exotic stuff in a second order.

Yes, it seems to be limited. 

I think it may just make sense to get the 2 row starter package (I have to check to see if that is the bigger case) and build from there.

The wait time does factor in but I really like this format and concept (and investment level).

From the videos on YouTube I do feel confident it will eventually offer what I am looking for and likely well beyond.

Short Answer:

Buy a two-row starter system and add a sequencer and some filters, as a minimum, and listen and read a lot whilst you experiment.

Long Answer:

You really can't go wrong with the two-row starter system, both in terms of versatility and value-for-money. Yes, there is a waiting time, unless you buy from a dealer, but then you lose the ability to tweak it a bit (e.g. start with a wider case).

In my opinion, there is no golden formula for recreating a specific style, rather one way is to put together a great set of tools and then set about understanding them and how they interact. Along the way, you'll make some great discoveries that will be your version of your target. I also advocate a spot of immersion, both reading about and around your target, and also a lot of listening.

One crazy thing that I did was to drop a Klaus Schulze track into Reaper. Having worked out the various parameters, I hit record and soloed along. I then created a different backing track that matched those parameters and dropped my solo on top. As this was just an experiment in "soloing like Klaus", I created a few different backings.

Anyway, back to designing a modular. You can add a few choice modules to a starter system, but you don't need much to get going. The sequencer and some extra filter types are no-brainers, in my opinion of course. Building slowly can be frustrating as you have the aggregated wait times, but it does offer the opportunity to get to know your instrument, unless you are already quite experienced, in which case it really designs itself 

I also recommend reading something like "Patch & Tweak", if you haven't already. Alternatively, there are some great free resources to start to understand what modules do and how they interact.

Apologies for the long post, it's very much from my point-of-view. To finish, creating something of the complexity that you desire will require a very large system, a few instruments or much layering in a DAW. Or all of that...