Take heed of the mantra repeated often by R.Proffitt, that hard disks should be considered temporary storage. More long term storage is in CDR/W disks.

With that in mind I have noticed little to no difference in the quality of drives offered by Western Digital/Seagate/Maxtor. With the exception of IBMs Deskstar(Deathstar). The only difference lies in the length of warranty and the ease of obtaining an RMA.
All of those drives should fit into most external enclosures without much issue.