You have to choose the bluetooth device first and know about its category i.e BLE or Classic. For BLE you can use Corebluetooth and for Classic one you have to use MFI. Yes MFI will be paid and expensive too. Refer to developer site for this and also this question.
iPhone bluetooth and "Made for iPod"
https://mfi.apple.com/MFiWeb/enroll.action.
Hope this helps.