Or run OSx in a virtual machine, as I may or may not have done.
Anyway, devices sold through a third party can be registered fine in DEP now, but as said they have to be authorised by Apple, and it can be a frustrating pain if they can't automatically register the serial numbers on your behalf. I had to register our iPads in Meraki (I assume any MDM would do) then copy and paste each serial number into an email back to the reseller so they could register them on our behalf.
As I recall it was a multi-stage process for us - buy iPads, register for the DEP with Apple, wait a week or more to be approved, ask reseller for
their DEP number, wait for Apple to acknowledge their DEP status, register iPads in an MDM to obtain serial numbers, ask reseller to register on our behalf, wait 24 hours for the devices to be approved. Will to live successfully lost.
You can manage the devices through one iTunes account depending on how you want to play things - obviously they would share a cloud, the apps, etc. and I don't think it's supported by Apple currently, but it has some obvious advantages.
To register multiple iTunes accounts through one external IP address you will need to contact Apple to be whitelisted, as usually they will only allow 3 or 4 new iTunes accounts to be created in bulk.
I found the whole process of setting up these things utterly horrible. It took 2 or 3 weeks to get things moving
at all, and that was just to get to a stage where we could start testing things, though to be fair that was as much down to our reseller as it was Apple, but the whole system is still an enormous mess. I wish we'd gone with Chromebooks, as was my preference, or Android.