NAS system is really for two things:
1. For everyone in your house who wants to access the same files on a NAS hard drive, which is always on, rather than transferring the same file around via USB or the main computer have to be turned on for them to access the file via home network.
2. For you to go out, and use the Internet to access your own NAS which is left at home. Think of those NAS as kind of your own personal cloud storage, that only you can access when you are away from home.
Unless either or both of those points is something you want, then there's no point in using a NAS.
I would recommend that you do buy two external drives.
One should be left attached to your computer, but if possible one of those kind of external drives that comes with its own power supply, something like the MyBook series. Make this your main back up, where you always back up to regularly.
The second should be very more portable, and powered via the USB, something like the My Passport series, which you could then plug into the computer, and make a weekly or monthly (depends on how much changes you made to your data) backup. You just remove that external drive and leave it somewhere else far away from your home. (ie: with other family members, in your office,) For the purpose of a second back up in case your main back up fails or the house gets burned down.
By the way, it do not have to be USB, it is up to you which system you like, ie: Firewire or whatever. And it do not have to be the actual series, just whatever is similar to the series. ie: When I said MyBook, it do not have to be MyBook, just whatever other manufacturers got that is similar to the MyBook.