I don't think there are any official park-n-ride lots in DT Tempe, you have to go further east or west along the line to see that, like at McClintock and Apache, or at 44th and Washington. If you find an apartment on the north side of University, why not just take one of the free shuttles to the 3rd street station once the light rail starts?
As for commuting from Ahwatukee, why not take one of the
snazzy new Rapid buses instead? There's a park-n-ride lot at 40th and Pecos.
I don't think the monthly pass will go up in price a whole lot once the Light Rail starts.