As far as I know the Red Devils are looking for a goalkeeper as well after Onana started to make mistakes very often. This means they would need to spend a lot of money in total. As long as there is no problem with FFP rules I don't think they would refrain from spending that much money. Manchester United are known by splurging on players.
I think it would be better if they sign a strong defensive player as well. Because their defense is very vulnerable this season. Even Varane and Casemiro can't play like they were playing before.
If Man United looks for a new goalkeeper, there should be 1-2 goalkeepers to be out from Man United because Man United has 3 goalkeepers now. Besides Onana, Man United has Altay Bayındır and Tom Heaton. I don't think Man United will keep all the existing goalkeepers because it will spend too much money for goalkeepers' salaries. As long as they don't break the limit made FFP, I think Man United will have no problem to do it. But honestly, it is too early to buy new goalkeepers because they just signed Onana and Altay Bayındır in the previous transfer window.
Man United has too many problems, they have a problem with defense line and attacking line. I think both defense and attacking deserve to rebuilt. So, I think it is okay if Man United are looking for attackers. But they also need to rebuild the defense line as well.