Its not completely essential to have a probe/curette, what is essential is having tools to do a good PEP service.
For me I find nothing better then the probe for ensuring all the cuticle from the sidewalls is removed.
The tools I use are a double ended curette probe and a precision pushie. I also love my double ended cuticle chisel.
Whatever you need to get the job done most effectively.
The probe is the smaller of the two, for removing tiny 'foreign' objects and detailed cuticle work, the curette is larger and for removing cuticle from the nail plate. Both very good.
I use the probe more- use with care though so as not to scrape the nail plate with over zealous cuticle removal- particularly if using with cuticle away.