Watching your wife with another

There's an age gap between my husband and I and he's at a stage where he can no longer stay hard. He's tried meds but bc of the cause, they don't work. He pleases me in all the other ways. He has excitedly proposed inviting a 3rd. He wants to pick him out, vet and set boundaries, of course. He wants to actively watch and participate in my getting pleased by others. I'm at the point where, yes, let's try it.

My biggest concern is obviously disease. And a few other boundaries of my own.

Anyone with experience have any tips, advice or opinions?