I would say 750 to 1000 watt depending on price. You don't really need that much power, but it gives you plenty of headroom and the prices for PSUs are pretty reasonable.
You could encrypt your hard drive to protect the data. There is a really
good write-up on how to protect yourself in the forums.
I have a
NAS box to backup stuff so that I can grab it in case of a fire or something that forces me to leave quickly. Since it's attached to my network, my family can also back stuff up there. I can upgrade it and even run a website and/or media center off of it. It's very versatile if you have the time to figure out how to use all of the features. I just prefer this to an external hard drive, which is essentially what it is, but it's more useful.