as for buying balls, do not waste your credits. You should be able to make master using the starter ball.
I never started purchasing balls unitl I was a master, and then, I bought 10 credit sleeves. When I made TM, I went to 65 credit sleves and was able to complete and win a TM RG with those
The expensive balls, including the Callies, are not needed until you reach legend and are playing from the tips, needing the spin and distance