I'd say go for two or three oscillators. Three will give a good amount of sound making scope.
When you say one for the bass and one for the mids, do you mean them both to be components of the same sound? This way would make the most sense to me, for example one oscillator at C3, another a fifth above it, and another acting sub oscillator an octave or two below - the sub osc doesn't necessarily need to be a dedicated oscillator, see Gordon's tip on the frequency divider.
As you say, splitting the keyboard would be a bit harder... worth making a monosynth at this stage. I'd be inclined to say build up the basic monosynth you have planned, but make it all patchable for future expansion.
I'm gradually piecing the components together for my own DIY monosynth, but fitting it around all the other projects, a music career, a girlfriend, and necessities like sleep and food isn't easy!