I stay away from Manson. Most likely would cost too much for a guy I don't feel is a legit top-pairing guy. I feel similar about Dumba (barring a trade like Johnsson+Dermott getting it done).
Some guys I'd be targeting that are not on playoff teams:
Top-pairing guys:
Jeff Petry
Top-4 guys:
Connor Murphy
Damon Severson
Dylan DeMelo
Top-6 high impact guys:
Adam Fox
Colin Miller
Petry might be our best defenseman if we could acquire him.
Murphy, DeMelo, and Severson are all underrated with good impacts both offensively and defensively- most likely they are relatively cheap (compared to the Dumbas/Mansons) with similar levels of impact.
Fox is having a star season on the 3rd pair, the Rangers are dumb, and he's blocked by Trouba and ADA (who I really dont want on the Leafs). Colin Miller is having a rough season on Buffalo, but he was great on Boston & VGK for years. I assume it's just like everything else in Buffalo- it sucks. In the right scenario the guy has top-4 impact written all over him.