how impossible would it be to try to get Weigl?
100% impossible..Weigl is going nowhere in the near future, as he has just extended until 2021 so he is going to stay at least until 2019 .
The same cannot be said about Sokratis though..here its just a matter of money, but IMO Dortmund would ask at least more than 30m€, but more likely closer to 40m€ and maybe even more. He is a good CB, but I think he is a little bit overrated.
A German CB that has impressed me during the laast season is Niklas Stark from Hertha, but he has also just extended until 2022..I would keep an eye on him though.
Your old buddy Kempf is also developing nicely at Freiburg and could be a cheap solution now.