It all depends on machining capabilities and space you want to use.

In Gravity, IDO and Impact , the KosBoxs run,like the great example Dantomkia, on a Mod 1 gearbox. Up to 20 to 1 in Gravity.
Tough As Nails uses as a small mod(DP 36?) planetairy gear as first stage, the second stage is Mod 1.5

M2 used a Mod 2 drivetrain.

So I belive you can get away, on Bosches that is, with Mod 1.
Most important thing, make the gearboxes stiff.(but that goes for about all reduction methodes)