I am no the greatest technical guru on these forums but for what it is worth here is my view on your options,
1. I agree about the reliability factor. However you may find that 74Gb is just a little bit too constraining. Those video files will eat up tons of space. Raw Capture Files, Converted Files, Working Space and temp files, it all adds up. Will depend on the size of your projects I guess.
2. Shouldn't be a problem Just make sure that the disk is a slave of the master and operates at a minimum of 7200rpm.
3. I have an external firewire disk myself (160Gb) and have had no problems with it.
Good luck whatever way you go!