SetsulYeah, 2500 or 2500K doesn't matter, whichever is cheaper. Only the 2550K would be marginally faster.
You could just get the GPU first (could still take a look at used GPUs) and upgrade the CPU only if it turns out to be holding you back (especially min fps).
RAM might be a good idea. How many slots does your mobo have though? Would be great if you don't have to replace anything and can just add more.
As for CPUs so you're not limited to the 2500 here's a rough ranking:
3770(K) > 3570(K) > 2700K > 3550 > 2600(K) = 2550K > 3470 > 2500(K) > 3450
Below that I don't think the upgrade would be worth it.
The 2500(K) should be around 20% faster than 2100, more in OW and DOOM.
I still have 3 free slots for RAM and thanks for the CPU list, I will have it in mind if I upgrade my CPU in the future.