So, Thresher is a special attack that targets all models in melee range in one sweep.
The question I have is: in order to use Thresher do you need to have at least one enemy model as an eligible target? I.E. can a model declare a Thresher special attack without engaging an enemy model…
A Special Attack on a melee weapon is a melee attack. If we look at melee attacks, a model can make an attack against any target in its Line of Sight and melee range. As Thresher does not say it doesn’t need a target (as opposed to Thunderhead’s pulse) it would require a valid target, or enemy model.
To supplement Malkav’s answer:
Here’s what the attack timing chart has to say on the topic.
Step 1 is “Declare the attack and its target.”
Unless a special rule says otherwise, attacks need to have a target. Here’s what the rules say about valid targets:
Thresher does not say it can target friendly models; it says, paraphrasing heavily, “It sucks to be you if your friend is using Thresher nearby.” 
You still need a target to use Thresher (because Thresher doesn’t say otherwise); it just so happens that when doing so, you make attacks against everything in range, regardless of whether a model is a friendly or enemy model.
