Some general guidelines:
a) Hard Drives are not going to be super reliable. If more reliability is needed, backup is needed. The failure rate is around a few per cent. There are zillions of discussions about manufacturer based on user anecdotes. Some batches of the same model can be stinky. If you find one that seems well priced, check the reviews on Newegg or elsewhere to see if the choice is notorious.
Here's one recent report:
http://www.behardware.com/articles/843-6/components-returns-rates-5.html
It's better just to go with one of the major manufacturers (only 4) than to fret about which one. If "stashed away" means not used frequently and not bumped around, general chances of hard drive longevity are good. Based on reports, 2TB has been more reliable than 3TB.
b) Firewire option is consistently more expensive than USB only. For long & frequent transfers it may be worth the extra $30 or whatever. I agree with Dustman that USB 3.0 would be good to have. There's little price difference from USB 2.0 and more flexibility later on.
c) The $110 to $120 price range has been the on sale/bargain range. Deals come up all the time, listed on dealmac type bargain search sites, in stores, & at OWC, newegg, Amazon, etc. Dustman's suggestion fits.
d) The build your own approach has some advantage. You can get a decent enclosure and pick the hard drive (specs) you want, often with a better warranty than the package. But recent hard drive prices have remained high since December (Thailand flooding). It's hard to beat the complete external hard/enclosure sale prices.
