Haaland isn't the same type of player as Martial. We've seen how out team can't always create and that non creativity from midfield(especially with Pogba out) was offset a bit by Martial because he can link up play, technical and good on the ball. Yet even now we still to add to our creativity. You bring in a pure goalscorer and he'd struggle to reduce our lack of creativity in the middle of the park.
I say we sort out out midfield first than we can start worrying about bringing any type of striker because right now I think that a player of Martial ilk is more suited to help with the lack of creativity in midfield, and even in his case we still do struggle to flow properly if midfield doesn't step up(Brighton Fred is needed + Pogba).
Please just check out Haaland before you assume that he is this or that kind of player. I have posted some links to decent analysis of him. He does have excellent link up play, creates space for his teammates, can hold on to the ball, pass a defender, knows where his teammates are, has the vision to spot a good run and elite off the ball movement to give his teammates an alternative. He does have 6 assists this season and should have had the double if some of his teammates were better finishers.
About his finishing, I only have statistics from CL: 7 goals in 11 shots (best conversion rate in the CL for players with 4 shots or more in the tournament)
Some questions for reflection:
Do you believe we could win the league or CL with Rashford/Martial/James as the leading attacking line? Or do you think we need to get in a at least one world class attacker to get there? Will Martial ever be a consistent 20+ goalscorer in the PL? Is there a chance getting in a top talent and adding more competition in the attack, could actually help improving our current attackers too?
...yes, I know we need to upgrade our midfield too...