Offensively, Beltre moves from a park that murders righties to one that is righty friendly. He's had a big home negative / away positive split the past few years. I have not looked at his spray chart lately, so I don't know if he's a dead pull guy (I've seen people say yes and no, and have nto seen him enough to give a worthwhile opinion), but even if he sprays the ball a bit, Fenway helps righties with a reachable wall for doubles, a big gap in right center, and that weird RF fence that allows for flare/slices to wrap the Pesky Pole or scoot down the line.
By UZR last year, then he is 25 runs better than Lowell. One year result, but that's a lot.
The big thing that is going on is that, after a couple of years of declining payroll, the Red Sox are spending again and likely to pay luxury tax. They should have a payrol lin the $180M range. They have more than $50m coming off the books next year, before they try to resign Beckett and Martinez.