External drives are great for storage and backups, but they are slow to work on. I recommend getting the biggest internal hard drives you can for your editing system, keeping your current projects on those internal drives, and backing them up daily or weekly to external drives, which you then (ideally) keep in an off-site location.
You need to keep at least one and preferably two backups of everything. The best software for incremental backups to external drives (that I’ve found) is called Carbon Copy Cloner. It’s is a very inexpensive and extremely high quality piece of software that you can download from http://www.bombich.com. You can also subscribe to a service like BackBlaze – http://backblaze.com – which automatically uploads all your new files to a bomb-proof array somewhere. BackBlaze is a great idea, but since video files are huge, it can take months to backup your old files, so start with it as soon as you get your new computer!
For external drives, I’ve had bad luck with Seagate, and good luck with Western Digital (I get the Two-Terabyte “My Book” drives from Costco when they go on sale for less than $200 each). I’ve had a lot of colleagues who have had Lacie drives fail on them, so I don’t recommend that brand either.