There's nothing magical about the HDD unit inside an external hard drive. Then whole idea of enclosures is that you buy your enclosure and you buy separately the hard drive you want. Then you mount one inside the other and you have an external hard drive.
The advantage of ready-to-go external hard drives is you have no assembly work to do and they often come with a CD of software to set them up, and maybe backup software. If you assemble yourself, you need to follow instructions from the HDD manufacturer about getting started - particularly, how to Partition and Format the drive once it's running and connected.
One small issue with some systems comes from the connection system. In some slightly older systems, when you chose to use an eSATA or SATA port for connecting, you may have to install the appropriate driver for Windows to be able to work with the drive. That would be on a CD of drivers that came with your motherboard, or maybe with the enclosure.
Pay attention to two details about interfaces if you buy and assemble. The first interface is between computer and enclosure. Almost all have USB2 connections available, and most have at least one other alternative - usually eSASTA or IEEE1394a (aka Firewire 400). Choose the case with the connection systems you want. My mobo has an eSATA port, so I bought a case with both eSATA and USB2 connection ports. I use the eSATA mostly, but I could use it with almost any other computer with the USB2 port.
The other interface is inside the case - the connection between case and hard drive. Typically you can get either IDE or SATA (or more likely SATAII these days), but not both in one enclosure. You need to be sure you order a hard disk that will match the connection scheme inside your enclosure. In my case, I chose one with SATAII connections and bought a SATAII Seagate drive to mount in it.
So, chose an enclosure that connects to your computer as you want AND has internal connections for the hard disk you want. Order enclosure and HDD, assemble (easy), set up the drive and smile.
The Rosewill RX82-U has only USB2 connections, which are a litttle slower that eSATA, but close. Higher price, but it has good features - holds 2 drives, has a cooling fan and internal power supply.
The Rosewill RX-358-S also seems good by customer comments, although a few had problems. Has both USB2 and eSATA, lower price 'cause it's for 1 drive.
Thermaltake BlacX N0028USU is interesting - easy to switch which actual HDD unit is mounted in it, if you have several you plan to use. Its specs say it is USB2 only, even though some user reviews talk about eSATA.
I chose one with no cooling fan. I figured no fan - no fan failure and replacement problem. But that depends a lot on what drive you mount in it and how hot it runs. I have a 500 GB Seagate and it seems fine.
The Seagate drive you suggest looks real good. Great size, price, and buffer. Seagate makes good units and great warranty. Reviews suggest this one runs quiet and cool, which matches my experience and reports from others on recent Seagate products. Note it comes with a small jumper installed to make it work as SATAI (backwards compatability), and you can (not absolutely necessary) remove it with tiny tweezers to get it to SATAII state.