About a minimum preorder amount... hmm... maybe something low like 5 -10 EUR would be a good idea, to sort out orders that really are interested in it or not.
Absolutely essential. It'll really prove interest levels without too much of a financial commitment and reserve them a place in the "queue" (there's always going to be a queue). It would be best to hold these funds in limbo to facilitate prompt refunds if necessary, and obviously the pre-order amount would be subtracted from the final price when their Pandora becomes available.