If you wanrt to go cross-platform you would probably want cordova or xamarin. These let you build one app that builds for iOS, android and windows phone (with restrictions if you're doing something really cutting edge).
If you wanted to target just android or iOS I'd still consider using one of the cross platform options.
I find cordova much easier to work with than Apple's XCode and Objective C.
Should you use internal storage? Depends on what your app does. I would imagine you'd at least want to use internal storage if not an on-device database (sqlite or similar).
My best first mobile app would be the web site first. That hits all the target mobile devices in one effort.
Now that the web site is working we can pick a target. For most that would be the iPhone and Android phones. After that, tablets from the 2 current majors.
There are systems where you write once and hit both but I found many companies balk at the cost of the systems or buckle under the load of the free cross platform dev systems. So the direction you head depends on your company size, and how much you will invest in the project.
My suggestion is create and two application one in android and another one in IOS which can load webview.. create an responsive wesbite so it will load in theses devices. instert google analytic code in your website so you can get to know what type of devices are using your application. By this way you can show your customer you have mobile application in two platforms. If your application have log in or more advanced options my suggestion is go with cordova or xamarin it was simlar like html but you have deep knowledge in java to make it running..
For building cross platform apps, I recommend Phone Gap Or Titanium.To build native apps, Android- Google's Android development studio or Eclipse studio.Iphone apps-Apple Xcode .
Regarding internal storage, it depends on the specific needs,like if you the data specific to the application or can be accessed by other applications.