Storability / shelf life of a newly bought iPad in unopened original packaging
will the battery be fine if the iPad remains in its unopened original packaging for 2-3 more months?


You should be fine.

The official advise from Apple when keeping an iPad device in long term storage without being used is:


Charge your iPad battery to about 50 percent.
Turn your iPad off to avoid additional battery use.


(Sharing the screenshot of the Apple support document webpage, as it is no longer accessible.)


  


New iPads come packaged in this state. So, you should be fine keeping the packaging unopened for 2-3 months.

When a product is discontinued by Apple, it typically doesn’t vanish overnight from store(s) and it should be easy to buy one (also from 3rd party resellers). You may also find discounted pricing from 3rd party sellers. So, general advise would be to hold on until any announcement is made by Apple.

I'd advise to wait till around Christmas (or delay the purchase as much as you can) to make the best decision regarding model to pick and pricing. It's likely you may end up picking a better option (newer model with upgraded specs, same model with more internal storage) for similar price. You may end up finding some holiday offers as well.

Keep watching for any release announcements from Apple. If you wish to receive an email instead, subscribe to the Apple product announcement newsletters as outlined here.