I am amazed that people are still pushing this "we haven't spent enough money" narrative. You are right to criticise the Glazers, and if United had owners like City's owners then they would probably have spent more.
But...let's go from back to front. United have the best goalkeeper in the world. Real Madrid really wanted him. Their reserve keeper has 94 caps for Argentina.
Both of the starting centre-backs against Brighton were signed by Mourinho for £30 million+. They also have Jones and Smalling, who are both experienced England internationals. Even Rojo has had his moments, and isn't bad for your fifth choice centre back! Shaw was the most expensive teenager in world football when he was signed. Valencia is very highly regarded by Mourinho, and he didn't give any indication that he wanted to sign a full-back anyway, he wanted to replace the centre-backs that he'd signed himself.
Pogba was the world's most expensive player when signed. He's still the most expensive player in the Premier League. Fred cost over £50 million, Matic cost over £40 million, Mata was signed for nearly £40 million, Herrera cost £30 million. Mourinho also moved heaven and earth to keep Fellaini. And then they've got Pereira who is now involved again.
Sanchez was signed on an absolutely massive deal, Martial was the most expensive teenager in the world when signed, and will cost close to £60 million depending on clauses. Rashford is another player in that ballpark in terms of value. Ditto Lingard.
Lukaku cost £85 million, and is the most expensive forward in the Premier League.
This is a good and expensive team. Potentially good. If you can't make a team out of that then you are failing as a manager. Let alone should that team be losing to Brighton, and not only losing but torn apart in the first-half, and flattered by a 2-3 scoreline.