Just a few more questions for now regarding list-building. My sense is that it's not as fine-tune fiddley as WFB or 40K.
Nope it is prety much straight forward. Decide the size army you want to play, line up all the mandatory minimums and consider the opponent en terrain and type of game to fill up your remaining points.
So, does it matter that much whether I take Chaos Hounds or Chaos Marauders?
Chaos hounds count as Cav and 20 cm move, and are nearest a "disposable" shielding unit that chaos has.
Marauders are infantry, so they will like a table with some place to stake their tents....
How does one choose between Chariots, Silver Helms and Reavers?
Theming your army....... other then that, the SH are there to punch through a battleline, Reavers otoh are great to obstruct horde type armies flanks, and they allow more concentrated fire compared to normal archers.
How does one get to a right balance or mix of units? Or does it not have much impact "in game"?
Experience ...and as said above, you will need to cinsider your opponent(s), tabels and type of game.
eg. for a EuroGT tournament you define your core army and 2 seperate "attachements". Depending in the games you play with the full force, or with the core and just 1 attachement. This is the kind of mindset IMHO that you should always bring to the table, be flexible
To be honest, I'm not looking to min/max optimize. I personally like to field a variety models in "realistic" ratios as long as I'm not totally gimping my army.
The only min-maxing that WM has are those in the Army selectors. Assuming you base you games of those you should be doing fine!