I have an old iPad with ios 9.3.5 that's going in for an overhaul, meaning everything will be wiped clean - so I'll need to install version 2 again (I think). Can I purchase another version 2? or can I download another copy of the version I originally purchased?
If you already purched version 2, you can always install it on other devices. You will find it in the list of your purchases in the App Store: https://support.apple.com/en-us/HT201272