Arsenal always sign Danny Wilson on my saves and he always becomes their top performer, never went for him so i have no idea how much he's initialy worth sorry.
If you need an instant impact though, well looking at Gallas and Otamendi theyre both not very dominant in the air, i would go for a realistic signing that is cheap, my suggestion would be Christoph Metzelder from real; he has decent passing, dominant in the air and very solid, probably less than 6mill to pick up i would guess.
Or on the topic of cheap solid germans maybe Westermann from Schalke?