The PMI-1 is still a great buy at the right price and PPS can do plenty of work for you on it. Any differences between it and a P68 will disappear after a trip to Palmers. That is other than the feed tube, 15 rounds for the P68 and 10 for the PMI-1. With a speed chute then it is really not a big difference. The Palmers SS is a great marker but if you are just getting into the stock class world it is a lot to sink into a marker. The PMI-1 or P68 will let you step into the SC world much more affordably and will give you some room to improve on the marker. Keep your eye out for a good SC harness or at least some of the wrist holders for 12g's and 10 round tubes.
The Phantoms are wonderful markers and the fact that Mike at CCI has not had to make any significant changes to his design in decades is a testament to it longevity and quality.
Good luck with either one and welcome to the world of SC.