I'd recommend just getting the inexpensive Arturia Keystep. Just connect with a midi cable and then it may be used to polyphonically sequence your P12. The Keystep does not allow single note advanced by each key hit, as does the PEK.
Another option is to find an old Alesis Micron. It has a great phrase recorder that records anything you play including spacing, tempo changes. It will transmit that to any midi synth. I wish more synths had had this.
The Arppegiator in the P12 is better than most. It's not polyphonic. However, keep in mind, you may run the arppegiators on both layers A and B. This would allow two notes to be played at once with different tempos and spacing even. I tried using it like that, and is fun to play with, but not the easiest to predict what your result would be.