If hourbuilding towards a CPL course, use the iPad in ways that'll be permissible for the CPL test. If you do it otherwise you'll develop habits that whilst possibly good in general terms, will need to be unlearned at great expense during your CPL course.
Any kind of NOTAM / Met / Flight planning apps however should be entirely permissible during your preparation for your CPL test for use on the ground, so using those during hourbuilding seems entirely sensible.
I'd make a point if I were you of NOT using the iPad in the air - stick to PPL basics in the air, because it's those, on steroids, that'll be used in the CPL course. You will used electronic navaids on the CPL course, but not GPS, and primarily for diversions. So practicing using those for position fixes and alternates will stand you in good stead - but remember that it's old fashioned DR that you'll need for the main nav component of the CPL.
I'm not an iPad user, but do use "Skydemon" Light on my laptop regularly for flight planning and find it very helpful. I also regularly use "Aviation Tools Free" on my Android phone and (via the bluestacks Android emulator) on my laptop. The latter is super for getting ICAO approach plates and airport data, as well as airport NOTAMS and met - so if you can find a way to run that on your iPad I think you'll find it really beneficial.
At work our Captains use iPads on the big research aeroplane, and swear by a suite of Jeppessen software that they have me spend several thousand quid a year on for them. I'm sure you can work out what that was quite easily, but I don't know if there are any cheap versions available.
G