1) If you want overclock, yeah, the performance among those 3 CPU is totally different. The 2500 can't be overclocked as high as the other two.
2) Maybe worth the update, but I would wait until BF4 specs goes live to check if Haswell or Ivy can run it.
3) Haswell-E would be expensive and maybe can give you only 2 or 3 years as max.
4) I would go with the h100i, if you case can handle off course.
If you're overclocking you'll want a "k" series CPU. More or less the only difference between the i5 and i7 is hyperthreading, the i7 has it and the i5 doesn't. Generally it's better to upgrade your GPU, but your build is more or less well-rounded, so either one will be fine. I wouldn't bother with Haswell-E. And I'd go for the h100i, you also want to mount it in the front of your case.