You can't mount the hoe without the rockshaft assist mounting plates. The backhoe pins to the plates, and the plates bolt to the tractor. I purchased my 3320 used with the 447 backhoe installed, so I can't say if the hoe comes with the plates or not. I would imagine that each line of tractor has its own set of RSA plates, so I would think that it would be a separate line item even if there were no additional costs. Deere may sell the adapters separate for the user with multiple tractor models, so he could move the hoe from unit to unit if needed.
And you will also need the power beyond hydraulics installed too. Your dealer will know all of these details.