Goalkeepers are difficult to scout because you really need to see them week in and week out over a season or two to know their strengths and weaknesses.
That said, I have to say I haven't been overly impressed with either Horn or Karius when I've seen them this season. I
Lots of Barca supporters say the same about Ter Stegen.
I think our best bet from the Bundesliga at least is Bernd Leno. Still very young (24) but has loads of experience - been Leverkusen's starting keeper for 5 full seasons now, which includes extensive experience in Europe as well.
I think his profile is ideal - he's played for a relatively big club, is experienced enough not to be just a flash in the pan, and also has room to get better.
He apparently has a 13 million release clause, which isn't terribly expensive either.
Of the other suggestions, I think the Sirigu one is worth considering. He has been wanting to leave for first team football since January, so he'd definitely be available.
He also had the reputation of being a very solid, error-free keeper in France - perhaps he could be a van der Saar type character for us?