Given how many teams are in the market for the top-tier FA closers - Chapman, Jansen, or even Melancon - including the Dodgers, Yankees, Nats, Marlins, among others, I just don't see that the Sox are going to sign one of those guys to team with Kimbral. Hence the interest in Holland. But I think they're probably looking to get at least one other guy to deepen the pen, whether its resigning Ziegler, or signing another set-up guy (or guy with set-up potential) - Keith Law mentioned Daniel Hudson and Luke Hochevar as possibilities here. Or there are trade options like Wade Davis/Kelvin Herrerra from KC, Colome from TB, Andrew Miller (though I suspect the acquisition cost would be prohibitive) or David Robertson from the White Sox (or Nate Jones).
But I agree that strengthening the bullpen is the first priority. I certainly don't see them spending $90-120m for 4-5 yrs of EE. Beltran seems like the first choice for a bat, which makes sense given the likely cost and length of commitment. Not sure where they turn if they lose out on him - Moss? Or explore the trade market - Frazier? And while I think there's always the possibility (20% chance?) that DD will pull off a "blockbuster" deal for a big name (Sale, Verlander, or my favorite, Goldschmidt), I hope that if he does it doesn't include more than two of the potential high-ceiling assets that always get mentioned: Bradley, Benintendi, ERod, Moncado, Devers, or Kopech. More than two of those in any one deal would be an overpay, IMO, although there are other valuable pieces that could be added, depending on who's coming back (for example, Goldschmidt > Sale > Verlander, in terms of what shoudl be the acquisition cost, not that any of these are likely to happen, in part because the sellers are likely to set the price higher than we should be willing to meet).
A post at Over the Monster discussed, and rejected, the possibility of a deal for Miggy.
http://www.overthemonster.com/2016/11/21/13697516/miguel-cabrera-trade-red-sox-tigers . I agree with that assessment, given the back end of his deal, unless they are practically giving him away, which I doubt they would.