We're not lacking in capable CB's, we're lacking in overall depth and a solid defensive system. Throwing random money away for a "world class" defender isn't gonna solve any long term problems or even be a good fix. A guy like Martinez is fine as a transfer because he's reliable, adds depth, and I wouldn't be surprised to see him starting and performing well and helping to fix some of our actual problems.
Buying Hummels would just be buying a big name to give satisfaction to paranoid fans who don't think further about the problems. What would come from buying a slow sweeper-stye CB who gets killed on the counter (something which he doesn't have to deal much with at Dortmund)? Honestly a guy like Marquinhos, despite being much younger, is better fitted to what our defense faces and currently needs. All people are doing is judging the defenders off of popularity and age and not actual playing style and quality. Thiago Silva is a good option obviously but he's not a long term solution, would get injured a lot (which is the last thing we need right now), and has an insanely high price tag and wage. Buying Martinez/Marquinhos is not as bad as people make it out to be, they just freak because they are not the famous high profile names that they want to see at the club.
Puyol is 35 and injury prone, Mascherano is not a real CB, and Pique is inconsistent (though I think getting him the right partner could really help this), and I'm not sure he'd pair well with someone inexperienced like Marquinhos or Martinez (though Martinez is pretty similar to Puyol, he's still fairly inexperienced).
I understand your doubts about Hummels, but it doesn't have to be him. I think Silva is the best option, he fits what we need perfectly, and he's a proven world-class defender. Not to mention he isn't THAT old, especially for a defender (imagine buying Puyol in 2007 or Xavi in 2009, not to mention we bought a 28 year old Villa for 40m in 2011 and he helped us win the CL). I don't know too much about his injury problems, so I won't comment too much on those, but I agree that those could be a problem.
I think someone like Kompany (who we were also linked with, though the rumors have died down) would also be a good choice. Or maybe Vertonghen (though we haven't really been linked with him). IMO, we need to get a proven world-class CB to be the starter and then (after we've made our other signings, like GK, etc and have money left over) consider buying Marquinhos or Martinez as a backup. Like I said, they'd be good backups, but we need a proven world-class CB as the starter for numerous reasons. For one, I don't think Pique would pair well with an inexperienced CB like Marquinhos or Martinez (like I already said). Another reason: It can be argued that we don't have a single world class CB (I'm not saying that's true, but the argument can definitely be made).