The potential impact Sanders has when he's healthy is probably far more significant than the potential contributions of a 6th WR or 5th OT. By bringing him in on a low-cost contract, even if you give him a roster spot on opening day and his salary is guaranteed, it isn't a huge hit if he goes on IR.
As for what Sanders will be willing to take, we'd have to ask him. But he already got paid big in his last contract ($20M guaranteed), so he might be willing to take less to play for a winner. You're probably right that he'll take a big money offer if it's available. And with the draft not being particularly deep in terms of safeties, he might get that big offer. But if he doesn't, I wouldn't mind kicking the tires on him for the vet minimum.