can i trade in my old iPad air after I buy my new one and transfer the data?

Yes.

If you use the trade-in process after your purchase your new iPad, if your device is accepted, you will receive an Apple Gift Card to redeem against a future purchase from Apple.

Apple Trade In - Apple

By contrast, if you begin the trade-in process when purchasing a new device, the agreed trade-in value is refunded to the Credit Card after your trade-in device is received and accepted by Apple’s trade-in partner. The trade-in return is timed to occur after you receive your new device. For this reason, many choose to use the trade-in option directly from the item purchase page when buying from Apple online.

If the purchase in made in an Apple Store, the Genius Bar staff will assist with transfer of your data - and process the trade-in immediately against the cost of the new device.