according to the RR Database all public safety agencies in Pima County are on a P25 system with some talkgroups using Phase II. the good news is that with the exception of the Marana Police all talkgroups are unencrypted. if you want to listen to Marana PD you're out of luck since no radio can decode excrypted signals.
a Phase I radio capable of decoding P25 (they all do) a Phase is a lot less expensive than a Phase II capable radio but you may want to consider a Phase II radio in case Pima County decides to upgrade. if/when they do upgrade and you have a Phase I radio you'll end up with an expensive brick.
i provide this link omly so you can get an idea of what's out there, their costs and features. once you decide shop around for price.
Scanner Master
i have the Uniden 536 and like it very much but you will be getting various recommendations. there is an Arizona users group on RR. you may want to post there for help as well.
AZ Discussion Group
good luck.