Done it!
By jove, it worked. It's important that you get the 12.6.5. version of itunes or it won't allow you to sign in. Then you purchase it on your computer as long as it's the same sign is as the ipad, and it sits and waits for you and asks you if you want the latest compatible version. Now...
I'm in a similar boat. I purchased a recon ipad for irealpro specifically. I really didn't expect compatibility issues (naively!). I'm interested in how you got on.