The biggest challenge you have is the order in which you did the project. How did you acquire the devices? Were they donated or purchased? Did you purchase direct from Apple and if so, do you already have an Apple Customer Number? Ideally, you would have had ABM setup prior to purchase. If you did, and the purchasing channel was aware of its presence, then the 1000 devices would have appeared in ABM automatically. From there, you could define an MDM token and assign the devices to the MDM for management.
The reasons I ask is that if you do not have ABM setup, then these units are unassociated devices (think retail activation). In order to get them into your ABM, you will either need to work with Apple to post-load them after ABM is setup (assumes you can prove purchase) or you will need to side load all of them using Apple Configurator. If you need to side load, please be aware that a user could drop it out of management during the first 30 days. Sideloading with Apple Configurator will require the tethered connection of every device to a Mac to allow for injection into ABM. Once injected, you would reassign to your MDM (as it initially appears in ABM associated to Apple Configurator).
Now, why would having them in ABM be ideal? There are a number of reasons but the key ones are that you will have the ability to automatically enroll the devices (once you get them into ABM and have an MDM), you will be able to remotely update them (which requires supervision), and you will have established chain of custody so if a device is lost or stolen you can remotely lock it to prevent unauthorized use. On top of all of that, you will also gain the ability to volume license the apps that you want to distribute. This can eliminate the end user act of adding apps to the device.
Once you select an MDM, you will be able to prevent the use of personal Apple IDs on the device. However, if you do allow them, you will want to make sure that the user cannot activation lock the unit. This is the process of associating the hardware with an Apple ID for the function of Find My. With the MDM you will be able to define restrictions and device behavior. And when the devices come back and need to be wiped, you can issue the wipe command from the MDM, allowing for rapid return to service between users.
Have you applied for ABM yet? Have you selected an MDM? This can be a very efficient solution if done right.
Hope this is helpful.
Reid