I know he'd cost Alot but hummels is ideal. Young talented natural cb, who is good on the ball. Tempting him away from Dortmund will be hard though, they don't want to let go of him. He would be the long term replacement for puyol as we know he is aging and having injury troubles.
I think when going for puyol replacement we should not settle for second best, as it's our long term future in that position, and puyols shoes are hard to fill.
We can't be compromising and Martinez would be a second best option. The position is too important and puyol needs a replacement without too much drop in quality
It's a bit like the signing of cesc, seems expensive and unreasonable at the time but he's Xavi replacement along with thiago (I know he plays striker but that's only because Xavi is still around, he is being groomed as his replacement, look at the Leverkusen game for example)