From my experience the Google Play Store will allow phones which just don't have the capability to download Ingress. Ingress is a resource hog. You will not only need a good Android powered smartphone, but you will want one with greater resources. Like some of the latest models from Motorola or Samsung.
A tablet??? I have a cool, awesome and powerful smartphone, why would I need a tablet?
Your cool, awesome and powerful smartphone is great as a scanner, but you will need additional resources. Prime example is the Ingress Intel Map. No matter how powerful your smart phone is, the Intel Map blows from a phone; including the mobile version. For serious missions you will need constant access to the Intel Map.
Additionally, if you are part of a local community and use some form of electronic communication for collaborating missions having a tablet will make things a lot easier for you.
Tablets with GPS and their own wireless internet connection work best. Tablets which you are able to tether to your phone and use your phone's data can also work. Be sure to check with your wireless provider's terms.
Okay, you have a car charger that's a start. But to be a real agent that will not be enough. I've said it a million times before. Ingress is a resource hog on your phone. It will drain your phone's battery very quickly. You will hit some target rich environments which are not car accessible like theme parks, large parks or even some urban areas where the freedom of being on foot (or even bike) will be more productive than being in a car.
I immediately bought a portable charger after my recent trip to Disney World. There were so many portals, so much opportunity, but by lunch time every day my scanner was dead. And when you are really addicted to Ingress there is nothing worse than being in a target rich environment and have a dead scanner.
Later this summer I went to Busch Gardens Williamsburg properly equipped and over the course of the day turned the park which was entirely blue to entirely green. From destroying, hacking (enemy portals), building and linking I earned over 160,000 AP on that trip. A real boost to helping me level up.
So, you will need more power. There are a variety of options out there from the barely useful like this mini USB Charger which may get you one additional charge.
To this monster system created by some Resistance agents which can power 8 scanners at once.
That's probably overkill for most of us. I got this one from Amazon. It has 4 USB ports of various power levels and can recharge my phone AND tablet multiple times making sure I have all the juice I need to have a productive day and even enough to share.
Others get backup batteries. That works too, but I find it more of a hassle to take off my cover, and switch batteries than just plug into my portable USB charger.
Were out there walking, running, riding around with our scanner in our hand and not always paying the best of attention to the world around us. We have our nose stuck in our scanner as we try to get either close enough to a portal or get that optimal position to deploy or destroy resonators. Because we're not paying the best of attention, we slip and fall, we drop our scanners, we bump into things and any other number of things. Make sure you have a good protective case, not just a cool decorative one. While Geocaching once with some friends going over some unforgiving terrain I had a friend drop his phone onto a rock and bounce into a puddle. Needless to say his phone was then FUBAR. Quite an expensive accident.
First and foremost; do not play and drive!
But, having an auto mount whether it is a vent, dashboard, or windshield mount is a great tool to have. It assists in navigating to unfamiliar portals or having quick access to hack, deploy or attack when stopped or parked.
There is a a stoplight on my route to and from work that for better or worse I can go a week or two without catching the light. Only in Ingress are you wishing to catch a light on your way to or from work. But when I do from my windshield mount I can get them both.
Also, in urban settings where you frequently stop park and hack or attack, then move on to the next target.
You may have seen +Google Glass in some Ingress Reports. I specifically remember seeing +Klue S. wearing one in one report, but they pop up from time to time. If you are not familiar with it, check it out here: http://www.google.com/glass/start/ . You also see +Brandon Badger wearing one if you follow him on Google+.
I'm not certain how it works with Ingress, but man I'd love to get my hands on one.