My biggest concern is, we knew he needed surgery. Why not pass on him like other teams and get someone else. Are we falling behind Car when it comes to player evaluations?!
They didn't pass because their doctors apparently thought that he would be ready for the season to start and they negotiated financial protections for if he missed time. Plus, like you said, they are desperate for a pass rusher and this was probably the only guy left that might be good enough to actually make our pass rush decent. If he's not healthy then you really lose nothing but some money this year.
And I mean, it's not the same but they did the same thing when they took a risk on Brees when Miami's doctors would not clear him. And that was a 4 year deal IIRC. They did it because they were desperate for a QB back then and they are desperate for a pass rusher right now.
Car has no shot at the division no matter who they have at pass rusher so they had no real reason to take the risk. IF and its a big IF, the offense improves under Kubiak and Young is healthy and can get 8 to 10 sacks, the Saints are very much so in it for the division so it's worth it to them.
The only other real option was Clowney and I'm not sure if he wants to come here and at his age, I don't think he has the upside of Young. Of course, he also doesn't have the injury downside.