Hold-off on buying an iPad, it is not a supported device currently.
You're options are https://support.google.com/stadia/answer/9338946?hl=en-GB :
Which mobile devices (tablets and phones) will support Stadia at launch?
At launch we will be supporting Pixel 2, Pixel 3, Pixel 3a, and Pixel 4 phones for mobile gameplay, tablets running Chrome OS, and more devices will be made available in the future. However, purchasing games and managing your content can be done from any iOS 11+ or Android M+ device that can run the Stadia app.