Bandit
It's a bit like going from GCSE to A levels to a degree - some assumed knowledge, some common areas, some 'well it's not quite like that', some new concepts and some delving into far more detail. If you are going modular then you would need PPL and 150 hours before you can enrole on an ATPL DL course; therefore, yes, having a PPL helps.
I did all 14 subjects in 11 months but frankly it was a relentless slog and it hurt on top of a job and family commitments. I wonder if I wouldn't have been better taking longer and 'enjoying' the experience.
RD