104hp is not a must - it may work out better value ($/hp) and stave off buying another case by a few months in the future, though
Personally I always recommend 6u 104hp as a good starter size
This will also allow you to buy an actual MI Rings - so the person who created it actually gets paid!
And I would go for an MI Marbles over a Turing Machine
Otherwise, just get your minimum viable synth (4 or 5 modules) and then go slowly, ignoring what you think you want now, for what you actually need once you've started patching - more utilities probably
"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