Regarding the number of successful players, I already addressed this in a previous post, not specifically about Dortmund however, but the German League in general:
To say he is unproven well there is a lot of information which suggests the contrary. He has over 100 appearances for Dortmund in three seasons, scoring 34 goals. He has 11 appearances for England and was part of the U17 World Cup winning team and third place for UEFA Nations.
In my mind it does not make sense to say that just because he is from Dortmund he is going to fail and use two previous players as an example. You are correct that Kagawa and Miki failed. However, they failed because of certain traits of the players and the conditions at United, rather than anything about Dortmund specifically. With regards to Sancho, he knows Manchester having lived in City, he was raised in the UK so he knows about the culture here as well. The only question mark is whether he can adapt to the league, but you can say that about any player really.
The price is high, but he is one of the best young players in the world and he deservedly commands that fee. Hazard went for 100m with one year left, Coutinho went for an absurd fee as did Dembele. Actually, having thought about it, spending £80m plus some add-ons or whatever seems reasonable to me. You could argue maybe we are spending too much post-covid, but you have to trust that the financial people at the club will budget this accordingly and there is no reason to suggest (not that you did but just saying in general) we are going down the path of a Leeds and spending beyond our means.