That's an interesting way of looking at it - get the older versions now before they are upgraded for iOS 7. And, it'll probably work, as so as you are aware that you'll constantly have upgrade notices on your AppStore icon. But, if the un-upgraded apps are on your iPad, then they should work fine.
In addition, Apple has started a process where people can download the app version that works with their iOS, regardless of which version in current in the AppStore. So, say you go to download an app that has been updated and now says it only works with iOS 7. Before, you just couldn't download the app. But now, you will see a pop-up that says there is an older version that is compatible with your iOS and do you want that one. It's not perfect, as developers do have the option of ensuring that old version is not available at all. But, still better than nothing for most apps.
So, between you getting apps before they upgrade and the new AppStore process, you should be doing pretty good with apps on that first generation iPad. Good luck.
Marilyn