I suffer from Basilar Migraines and they are quite scary. I have done a LOT of reading on this subject over the years and your daughter's experience is NOT typical. While she shares many of the symptoms, the speed in which she blacks out is a concern. It should take minutes, not seconds. And usually she should have a clue as to the blackout coming on: slurred speech, dizziness, light-headiness, blurred vision, aphasia, confusion, flashing lights, strange odors or sounds (migraine auras). The numbness on one side of the body seems to sound more like Hemiplegic migraines as opposed to Basilar. Take a look at: http://www.healthcentral.com/migraine/types-of-headaches-38571-5.html Either way, they sound like they suck. Epileptic preventatives are the norm for both cases. Triptans are a no go due to the increased stroke risks.
She is of the right age (18) and gender (female).
Go out of your way to find a migraine specialist. They can be rare in some areas but worth the look. I would also post your inquiry on the migraine pages of health central. They have some of the best, dedicated readers and specialists out there.
Lastly, a caveat. I am NOT a health specialist. All I can say is that your daughter's symptoms seem atypical to what I personally experience and all that I have read over the years. Seek expert advice.
Good luck and do not despair.