Why would you ever want to do this?
Especially now in 2020?
Getting older third party apps for iOS 9 and 10 and older iOS versions is becoming an impossibility, now!
As of the beginning of 2019, many, MANY third party app developers are all more quickly and completely dropping older app support for all old 7, 8 and 9-year iPads that are running 2-3 year old and earlier iOS versions.
Third party app developers are removing all of their older, legacy versions of their apps from Apple's App Store servers, now.
Older versions of apps are going to be gone for good, now!
Getting older versions of app for these old iPad models, initially from Apple’s iOS App Store is getting to be very difficult and problematic, now, and will only be getting much worse in 2020.
These old iPads are now reaching their complete end of life and support.
Old iPads that can no longer be upgraded/updated will have NO MORE APPS AVAILABLE for initial download, at all, soon, from the Apple iOS App Store, for such old iOS devices in the very near future.
Older versions of apps for older iPads will be completely gone, very soon!
This is now the time to seriously consider a new or discounted, refurbished iPad from Apple or other Apple authorised resellers.
Not purchasing an old iPad that is going to be unable to obtain any more older apps from the iOS App Store and will soon become an obsolete piece of mobile hardware.
What you want to do doesn’t make any sense logically or monetarily in 2020.
None of what you stated here makes much sense.
Why would you want to purchase such an old iPad with an over 3-1/2-year old iOS version, just to go and attempt to upgrade to its maximum iOS 12.4.x, anyways?
Why not just simply purchase an iPad Mini 2 or 3 that already has iOS 12.4.5 on it?
Or better yet, just purchase a new or discounted, refurbished iPad from Apple or an authorised third party reseller?
At any rate, you are stuck with iOS 12, now.
Apple has no procedures nor provisions for downgrading/reverting their mobile devices to a previous iOS and iOS 9 through iOS 12 will never be re-signed by Apple for any iPads that can run a higher iOS/iPadOS version.
Best of Luck to You!
