In terms of of gettable strikers, right now I'd most certainly take him. He's a proper striker who stays in and around the box, and is capable of causing trouble with very decent close control and an intimidating physique. I don't think he's good enough to be first choice at the level we should aspire to, but right now he'd be a solid upgrade from Rooney, and I believe he's good enough to be an rotational option going forward as well.
I also think we're desperate for a wide player, preferably one on the right side, who can actually beat a man. Mané is a decent shout. Like Ighalo, I don't think he's good enough to be a first choice at the level we should aspire to, but he'd also be a very decent rotational option even if we were able to get there.
Unfortunately, given the dire straits we are in, I don't think we have a choice but to try and break the bank for players of that calibre right now, as soon as the window opens, in an attempt to elevate ourselves back into contention for top four. It's not ideal, but it is what it is.
There are probably cheaper options on the continent, and possibly elsewhere as well, but the key is that right now we need players who are ready to step in and contribute right away. The highest chance we have of finding someone like that is to overpay for someone who have already proven that they can do it in the PL.