How to see what apps you have bought after doing a clean install
On your iPad log into the AppStore with the Apple ID you used to purchase the Apps. Then go to the Purchased tab and select them to be downloaded again.



If you did not make a backup, which iTunes would have asked you to do before upgrading, you will lose all your data however.

You can also do this from iTunes by logging into the AppStore and clicking on Purchased and then selecting Apps under purchased: