I am all for hard drives as a storage medium, as generally they last longer than DVD/CD backup media (I have CD's from about 3 years ago that I am unable to read anymore because they went "bad").
You should be able to fill them up without a problem. Using NTFS. Since you are going to be filling them sequentially fragmentation should be really low, fragmentation occurs only when the device is moving files around, or removing them and writing new files to small but contiguous space, and spreading the files around.
There are certain limits that you will want to watch for, but Windows will warn you when you approach them, I believe it requires at least 500 MB or so free space.
As for 10% free space for disk crashes? You are joking right? If the disk is crashing it is time to buy a new one 😛. All the new hard drives have extra space on them that is available to write re-allocated sectors to. However SMART will warn you if there are too many re-allocated sectors as that is generally what happens before the drive fails miserably.