You have to upgrade somewhere. Your best players play LF, C, and 1B. I don't see Bourn being moved, so CF is set. They have Ramirez and Lindor at SS, so that's set. It appears Kipnis will be the 2B, so that's set. You can't count on Swisher in RF, so DH is set.
That leaves 3B and RF. You have to upgrade at least one of those spots. You have Alex Rios, Torii Hunter, and Michael Cuddyer as options in RF, with Pablo Sandoval, Aramis Ramirez (possibly), Hanley Ramirez, and Chase Headley at 3B. Are any of those free agent options better (taking contract into consideration as well) than acquiring Beltre? Possibly, but I think he's the best that is out there. I like Lonnie, I really do, but if you can grab Beltre in a trade I'd give Lonnie ABs backing up at 3B, 1B, and DH, or start working him out in RF.
You have to upgrade somewhere if you're serious about contending, and that somewhere looks like it has to be 3B or RF.