For the current Real Madrid team, financially Osimhen might be a very good option but his is very injury prone player. Mbappe is the best stable long-term player but he will drain their budget. I think that Haaland will be the best option for them.
I've been able to watch Victor Osimhen right from his days as a player of the Nigeria U-17, when he went to Wolfsburg, Lille and his now at Napoli, and can authoritatively say that Victor Osimhen isn't an injury prone player. We saw him last season what he did with Napoli and the number of goal contributions he provided to the Italian Serie A champions yet he wasn't injured. It was earlier this season that he picked up injury during one of the international breaks when he was playing for Nigeria but it didn't take long before he came back to full fitness.
If Real Madrid can buy a player like Victor Osimhen, I think it's going compliment their current fast football because the Nigeria Super Eagles player is a very fast player with speed and spacey abilities. Erling Haaland wouldn't ask be a bad idea but his footwork isn't as good as that of Osimhen and that's why I think Osimhen will make a better striker. That being said, Real Madrid doesn't eve look like they want any of those two players rather their attention is on Paris Saint Germaine forward Kylian Mbappe.