It all depends on Tarasenko's status. If he's back before the regular season ends, they can't really add anyone because of the cap situation anyway. If he's not back until the playoffs, they've got some room to maneuver.
Kreider is the name you hear the most, though he'll also be the player most in demand, so his cost will be highest. And Colorado also figures to be after him, so that would be another plus to getting him, namely keeping him away from a rival chasing you. Tyler Toffoli and Jean-Gabriel Pageau also figure to be on the blocks.
Guys who are more than rentals will take a higher cost, and likely someone off the current roster.
As to who the Blues might give up, look at what they got when they traded away rental players at the deadline in recent years. Stastny got them a first-round pick and a prospect, Erik Foley. Shattenkirk got them a first-round pick, a conditional second and a prospect, Sanford. So figure the deal looks something like that. A pick or two and a prospect, though I think they would want to keep Kostin. Could they trade from their defensive excess? They probably can't use Mikkola, Walman and Reinke (though if Pietrangelo doesn't re-sign, spots will be open in the near future).