The current iOS v11, although it still may need to be updated if an interim update has come out.
If you back up your current iPad contents to iCloud, then you can download it all to the new iPad.
I don't know about your iCloud storage, but I assume that you will just transfer your account from the old iPad to the new one. (Actually, I don't know whether iCloud is attached to a device or just an email address. Perhaps it is attached to an email, in which case there would be no effect. Just set up the link in the new iPad.
Hope this helps.