I don't really have musch experience of external drives other than LaCie, and even then, it's only the firewire400 versions, but even teh 400 versions seem reasonably fast.
As for the drives supporting HDV, I may be wrong here, but I'm assuming that's irrelevant anyway. As far as I know, seeing as HDV can in theory use the same miniDV tapes as SDV, the bandwidth of the video is the same, but using a slightly higher level of compression to fit all the data in, which means it uses the same amount of data, so yes, it should be ok. The only bottlenexk with handling HDV would come from the performance of your PC itself.
But it's early morning, I may be talking nonsense