I think Martinez is overrated as a defender. Matter of fact, I am certain he is overrated PERIOD on Bluemoon. He is a very good player, don't get me wrong, but he is not special. Just very solid. A Spanish Micheal Carrick if you will.
So he will help with game flow and play, but I am worried at the amount of belief so many have in what he will bring. Which really is a slightly more technical, Carrick with slighly better heading ability and drive.
Should we get him if we can? Sure, he is a very good player, but he probably is less a necessity than many believe.
Now to the OP" We have made poor purchases at the CD position: Kolo, Lescott and Savic were all poor decisions. Lescott at 24 million was just disaster of a decision. Kolo at 15 ( and not 12 as the OP listed) was also a terrible decision. Paying 15 million for a player Arsenal was benching that season was just awful, and then Mancini shelling 7 million on a defender who isn't ready is just ridiculous.
These decisions are not unique, except in one regard: We have failed to keep trying at CD, like we have at forward.