The problem with trying to pluck in guys from Germany is that EPL is very different. It's really hard to predict how commanding a player will be in the box. Karius has looked deer in the headlights since he arrived.
That being said, Trapp would be worth a flyer. Although I suspect he might not be a significant upgrade. I'm all in on Oblak. Liverpool have a strong nucleus of young players. They can afford to overpay to solidify the back (VVD and Oblak).