If no one else has an answer, I would say buy one on the compatibility list if you're solely going for Wii compatibility. That way you know it works and you won't have wasted money/won't have to return one you thought worked.
in my experience, the compability issues related to the HDD spin (rpm)
ive been use internal HDD with USB adapter, sometimes wii detect it, sometimes not.
so i use stabilizer and it has no problem anymore.
be sure what inside that casing. (maybe toshiba, WD, seagate, etc)
The new WD my passport drives that are usb 3.0 compatible don't work very well on the wii.
They don't work very well under linux either, and windows xp keeps insisting for the install of a special driver.
And you can't remove the drive to install it in another enclosure. The usb interface is integrated in the drive electronics.
So, maybe the older ones were fine, but I shouldn't recommend them anymore.
On the other hand, a western digital 2.5 inch portable pc drive in a low cost usb enclosure works fine.