Is a PP'er necessary? In my opinion, no. Give yourself a little time to learn the 250 and you will be fine. It's all in trial and error, and each and every time you go out there, you will bring back with you something new learned, to the point where you will feel like an expert at some point or another, but no matter how many times you go out there, you will learn something new. My advice in learning to PP with the 250 is simple, first, forget about the PP'er altogether. Try to get as precise an idea of where the object/coin is just by sweeping the coil over. Keep in mind that the machine rings off not in the center, but the outer coils. This would apply even more for a 150 without the PP. Believe it or not, if you get good at this method, you can PP with relative accuracy on that alone. Once that is put into practice, start getting good on the PP'er. My absolute advise for PP'ing with the 250 is do not even try looking at the screens bars. You will loose yourself. Do not even try to concentrate on a certain spot on the coil. Simply focus on the center of the coil and where it sounds the loudest as you pass it over the target. Although standard to change the X angle by sweeping forward and backward, I strongly advise against it and instead recommend you turn your body 90 degrees at give it another sweep. Listen, the advise me or anyone here can give you does not make up for actual hands on practice, so get out there and put some hours into learning your machine. It's no science and you will pick up its behavior.
Best,
Ricardo