Rangers getting St. Louis on what would operate as a 2 year deal would be great. Doesn't lock down money long term, gets you an elite playmaker from the wing, etc. He's older but he makes the team significantly better in the short term and, because of the short contract, the deal can't hurt you long term. Getting him for Callahan (or assets we use Cally to acquire) would awesome. I'd prefer that to Kesler.
Kesler is tricky. He's basically like Callahan but at the center position. Recent injury concerns, looks like a 50 point player, etc. He, like Stepan, is not a true 1C but rather a very good 2C. If you had Stepan and Kesler up the middle, your top 6 would have good but not great play up the middle. His deal is pretty good, money wise and year wise. I wouldn't mind Kesler but not if he requires moving big pieces. If I'm moving big pieces, it's for a TRUE 1C that can pass like Spezza.