If we move a forward I'd be fine with Tkachuk or Brown being brought in. But barring a forward trade I feel that Sergachev should be our pick. IMO not much separating any of these guys, and we have never had a defenceman like Sergachev in our system.
Not to single out a single point in your post, but why does it matter if we move a forward now? I imagine at best Brown would need a couple years before he sniffs the NHL and Tkachuk is probably at least a year away as well.