Several CM/DM types somewhere in the region of 30-45 million (Palhinha, James Ward Prowse, Alvarez, I'd put your very own McTominay towards the bottom of that range as well) Zakaria seems quite likely at this point as you say so Zakaria plus one of those above, probably an outlay between them of 50-60 million there. We are after another LM as Moyes wants a more direct goal scoring player there (thats why we were linked to Barnes, Newcastle did want him but thats gone dead) seems like we are budgeting 25-40 million there as well and then another CB to rotate/start for/with Zouma and Aguerd.
Maguire would IMO fit into that last category, and personally I can see some real merit to him coming here, but I don't think we have the budget for silly money, our budget for all the other options back there (Luiz, Tah) is somewhere in the 20 million range. I think that gives you a rough ballpark of what West Ham can spend in that position, 20, maybe 25 at a real push. I suspect the wages might be a bigger stumbling block mind you.
A loan to buy however and given our FFP position will be better next year with more Rice money AND higher profit in 2023 vs 2019 (which is currently counted due to Covid) means that 20 million might stretch to 30 next year instead, so it probably makes sense for Man Utd as well financially, to get more a little later on.