Yes, of course - it can be easily done. One way would be to have the user purchase the in-app purchase item, check for confirmation and then start downloading the audio track from your server. Let me know if you would like to discuss.

It really depends on the type/functionality of the application.

If the app is quite small and you miss out on providing the core feature set to the user, then that will not be a good user experience. In this case you should get 99% (you can never be 100%, there's always more you can do) before you launch the app. On the other hand, if the functionality you plan to leave out does not hinder the core user experience, but possibly provides a value-add on top, that is something you can add later with an update.

Also, if it is a game which has several levels/stages, you can add more levels and stages later and provide the user enough to play with version 1 to keep them busy till the next version launches.

Remember, generally the last 10% of fixes in an app take over 50% of the time, so you want to make sure you are not bogged down with minor tweaks and fixes if they do not hinder the user's experience in using your app.

What framework you use depends on your experience and knowledge of the corresponding programming language. I would also recommend looking into Xamarin, which allows you to create native applications for all platforms while writing code in C#

I would be happy to discuss further if you would like to jump on a call.