I guess I rate Biglia more than you do. He is a compete DM who is a leader in the midfield, has Serie A experience and a decent playmaker. Plus he has worked under Pioli and understands his system. Its a brilliant combination if we want immediate results.
Julian Weigl is an outstanding talent, possibly the best young DM in the world. I was trying to be a bit more realistic with my wish-list, which should cost around 100-120m for all 4 players. While with Chinese money, we could afford someone like Weigl (who would cost upwards of 50m), I doubt we will be able to beat off competition from Barca, Real and City.
Biglia is more realistic, I remember during his time in Belgium he mentioned that Inter was his dream destination... and now he is having contract issues with Lazio. He will still cost 20-30m, which is very high for his age, but like you said for our "first priority" position we should not skimp. We have no midfielder who is even close to the quality of Biglia as a DM.