From my experience, building your own will cost you more for less and with one exception, getit29's reply proves that (and you still haven't put your time in or bought USB cables or paid for shipping). The Seagate 7200.12 appears to have put to rest the 1TB problems attributed to the .10 and .11 versions. I have never had a single problem with a Seagate drive or a WD Blue (read the reviews for ST31000528AS on Newegg). You can reformat these drives from PC to MAC but (and I might be wrong) I don't think that a drive readable by one will be readable by another without a reformat.
The iomega 1 TB I quoted is now gone from Newegg. Amazon still has them for the same $95 free shipping and lists it #1 in popularity for External Hard Drives. Sorry if I seem to keep beating the same horse but I did a lot of research and asked the same questions as you before buying.
If you do build your own, buy an aluminum enclosure that will act as a heat sink and make sure that you get good contact between the metal case on the drive and the enclosure, using thermal grease (messy) or some thermally conductive material (expensive). Heat is the #1 enemy of hard drives with "dropped while reading/writing" a close second. Absolutely do not buy an enclosure cooled by a fan. It will be loud and the fan will fail 10 years before the hard drive.
Newegg has a Cavalry CAXB3701T0 for $102 with shipping. It is a USB/eSATA reportedly with a WD Green Drive inside (your mileage may vary, the USB only version supposedly has a Hitachi). A similar version on Amazon throws in the eSATA cables for $107 with shipping.
If you want to go Big, Newegg also has 2TB drives from Hitachi, Seagate and Cavalry (WD or Hitachi) shipped for less than the $190 to build your own.
Again, from my research, you will not be able to buy or build a reliable external drive for less than purchasing a ready-to-go from Amazon or Newegg.