I think we should be looking at a strong, technically sound striker that can play with his back to goal and bring our wide forwards into play, as well as win headers in the box. Martial has done well linking up our forward line recently, but he's better suited to the wide role where he can run at defenders more and exploit the space behind the full backs. Greenwood even more so, since he's (at the moment, anyway) poorer in the air, has worse movement in the tight spaces a striker often has to operate in and more deadly from those mid-range distances around the corner of the box that the likes of Robben and Ronaldo have scored countless screamers from. I think he'll make the most of his career if he focuses on perfecting the goalscoring RW role. We saw from his cross to Bruno that he can create goals from wide positions with ease too, so he'll bring both goals and assists playing as wide forward. Rashford is the same. Struggles centrally against organized defences, but thrives when he can use his pace and footwork out wide.
We don't necessarily need a huge goal threat like Haaland or Lewandowski. The goals will be pouring in from Martial, Rashford and Greenwood anyway. We need a Benzema/Firmino type who is enough of a goal threat that the opposition can't completely take their eyes off him, but also knows how to open up opportunites for the more lethal team mates next to him to take the glory. I'm on record here saying I think Havertz could be that guy for us even if most see him as an attacking midfielder, as he's creative, very physical on the ball, a good finisher and tremendous in the air, but he would not be cheap or a backup. If we're going cheap there's only so much to choose from. You're either getting a pound shop version of Benzema or a good but aging striker like Cavani. I honestly don't think there's any need to look for a player who will mostl likely primarily be resigned to a bench role, now that our current crop of forwards are performing so well and we look to be spending the war chest on Sancho this summer. Ighalo's happy enough to just be here, so he won't complain, and has done his job very well when called upon thus far. If it's possible to make it permanent I would definitely be looking into that if I was whoever makes those decisions, and then set our sights for a first choice forward instead when the money is available.