MS RDP client minimum iOS version on Apple iPad
I would trust the Apple reviewers that iOS 8.0 is the minimum required.

Also, I would also plan on not buying any hardware that is 32 bit since we’re so close to iOS 11 and no more 32 bit apps.

Microsoft’s support is actually pretty amazing for iOS - so you could ask them if they plan on leaving older versions of the app available. Developers don’t have to take down all downloads when they remove an app version from sale. So you could buy the app “for free” now and then ride an iOS 8 or iOS 6 device for years by backing up the older app version to iTunes and/or counting on it being available for “legacy download”.

There’s no guarantee they keep the old version up, so you should plan on iTunes and buying new hardware if you are riding the “about to be obsoleted” line for used / older iOS hardware and versions.