IMO, we should take a chance on Gundogan. It's a bit of a gamble, but look at it this way. If it doesn't work out, we lose a bit of money. Probably something in the 20-25m region. Shouldn't be a huge deal. If it does work out, we have arguably the best midfielder in the world in that age bracket and one of the best "xavi replacements" out there. For a bargain price at that.
If we wait until next summer, we could be hit with the transfer ban. Not to mention that if he recovers, his price will be way higher than what it is now.