Builds apps. Fascinated with what makes apps tick, and what makes users love them. Devoted to making mobile a better place.
No. However there are a number of services that analyze Appstore data and attempt to guess the number of downloads for specific apps and categories. Most of the detailed information is pay-only, keep in mind for iOS apps, this is all based on guesswork since Apple does not release detailed data for the entire store.
For the education (free) category, an app that I worked on reached the top 25 with ~1,000 downloads a day. The bar is higher now across the board.
I wouldn't even be considering the possibility at this point. You are still refining the product and figuring out how to engage+monetize. It is hard to "turn down" users, but splitting your focus will likely be a mistake until you've got the core figured out. More platforms is much more work than you will expect, especially with UI expectations being different.
You may not even need those options when it comes to growth either, depending on your space. Instragram did very well being extremely focused and waiting quite a while for other platforms.
Finally, when you do start acquiring users on other mobile platforms, be aware that they will convert at different rates! Don't depend on conversion behavior similar to iOS.
I've got a desk full of various iBeacon hardware, and for the most part the performance profiles are pretty similar. The major differentiating factors to me are the enclosures, battery, and the SDK (if you're looking to use the manufacturer supplied SDK). Happy to talk more about the details of the hardware I've gotten hands-on experience with.
Some of the other answers here missed your true question of _what_ should be evaluated. I guide my clients around this exact problem. Even though the focus of my business is native apps, I have no problem saying "you don't need an app for that". With more details I could help you further, but in general you're asking:
Who (is using the app) - millennials and retirees use devices differently
Where (is it being used) - is it mostly going to be used on mobile? is it an extension of something else
What (kind of experience) - Web/Desktop/Mobile all have different UI conventions and limitations
That's really just the beginning! As you understand the idea better and the targets, it should become more obvious which is the better choice. If you want to call me, I can provide more detailed suggestions about an appropriate route to take with more information.
There are many different approaches with prototyping, and it really depends on who you have involved with your team. If there is someone that knows Adobe tools well, there's nothing wrong with doing it in Illustrator or Photoshop. Other alternatives:
Keynote (there are good templates available)
Personally, since I know Xcode well, I now prefer to make a functional prototype on the device itself (with fake data, etc) because with iOS7 the feel and motion of the app has become much more important. This is hard to convey through material on a desktop and laptop, and even being able to touch and swipe on the device itself irons out problems very early on. Touch zones are also important for user interaction, which isn't always apparent until users interact with it.
The bar for the app experience has increased dramatically, so be careful smashing something together in one of the cross-platform tools. This is especially important when going to multiple types of devices because the user expectations are different.
All that said, if the interface is fairly simple, definitely go for a cross-platform solution! Be happy to go more into detail with your specific app idea.