There's a few guys that jump out that could probably be had for not that much, one that comes to mind is Lauzon. A good stay a home defenseman, making only 2 million per on a bad Preds team. He'd be nice to stick Severson with and wouldn't cost much.
Another player that would make an impact at a higher level would be Pelech, good defender, not going to light it up offensively, but big and will play shutdown minutes. I'd say this is probably more of a mid tier option.
Lastly, my perfect candidate would be Dobson. #1 dman, young, good at both ends of the ice, could easily run his own pairing and would make our top 4 very solid.
That last option is more of the "dream" option as I doubt the Isles give him up easily, they'd want an overpayment most likely.
Someone mentioned Andersson, that's the better version of Provorov but a whole lot more expensive. Very good defensively and can eat big minutes, only issue is he's got one year left on his contract after this and will want big money. He'll also be 29 soon, the rumored price is a 1st, young player and prospect, that's probably too much for a potential 1 year rental based on where this team is now. I think any big trade will need to have some years under control, I rather try and go after someone like Dobson who would give the team more guaranteed years.