Cech and Sol were on frees and it hardly took a genius to spot that those two would work, although I would argue that Cech's replacement should have been a priority this past summer.
As for the rest, Kos was a good buy, Per was not, Monreal was a good buy, Mustafi is very questionable at this point, Gallas was a nutter and Sagna was also good.
Would you like me to list the failures at this point? Nah. Can't be bothered although I'd start with Almunia.
Harsh on Mertesacker over the entire period of his time with us. Perhaps this season is one too far but that is not his fault if so, he has done very well for us over the years and joined us at an exact time when we were footballistically ****ed.
Him and Lego head, perfect signings at the time.
Aside from the inaccuracies others have noted (your opinion of Per is ludicrous) you are moving the goalposts slightly with Mustafi. You stated that Wenger did things on the cheap and got it wrong- even if Mustafi is **** (he isnt) the point is that he wasnt cheap, far from it. If anything his being **** would only prove your theory to be even more inaccurate.