Some suggestions, re external drives, I've gotten:

Irrespective of brand name, it should have a Seagate or Western Digital drive inside; avoid Fujitsu.

3TB USB 3 drives with 16-32MB cache with 7200 rpm recommended (over Firewire or Thunderbolt).

Partition into 2TB for Time Machine and 1TB for other (possibly to mirror iMac HD in case it's required to boot from external drive — as though I knew what any of that means or how to do it).

Any comments?