I don't think I'd be too worried about the final two years of McDonagh's deal. A lot of the smart and technically sound defensemen age pretty well. We were happy to roster Bouwmeester at a similar age. On our team he might be the #1 defensemen but he still wouldn't be asked to do too much since we have a lot of strong players around him.
The other benefit compared to acquiring someone like Chychrun is the asking price should be a lot more affordable. Not only is he older on a more expensive contract, he has a full no-trade clause. The Blues should be one of the prime destinations for a player like him and Tampa Bay will have little leverage to ask for much more than salary relief.
Granted the Blues don't have a ton of cap flexibility either, but there are probably ways to make it work.
I also like the idea of upgrading the D without moving Perunovich. I know a lot of people don't want him in the same lineup with Krug but I really think he's got a lot more to show us on the ice.