One question I have for you is if you can parallelize any parts of your code? A quad-core CPU with hyper-threading will not be much use if all the activity is happening on just one thread. I know it's a simple stupid question that you've probably already thought of, but if it takes a month on one core, could it take 2 weeks if run in parallel? With such a long runtime, I'd be looking for any possible way to divide and conquer.
Ok, but I'm sure you've already thought of that too! The next thing I'd be careful of is your estimate of single-thread performance. When you say it takes a month to run, is this on the big cluster you're talking about? What kind of specs does that machine have? You'll want to look up the single-thread passmark score for that CPU in order to compare to one that you would buy. If the cluster has a single-thread passmark score of 3000 and the process takes a month to run on it, if you buy a CPU with a score of 1500, well then it might take 2 months!
Check out the single-thread passmark scores here:
PassMark CPU Benchmarks - Single Thread Performance
Here are the scores of some of the procs mentioned in this thread:
E3-1230 v3 = 2,100
E3-1225 v3 = 2,016
i3-4330T = 1,774
X5675 = 1,407
X5660 = 1,294
X5560 = 1,286
L5640 = 1,101
You may not be able to buy an E3V3 used for a good price, but keep in mind that even if you get a good deal on an L5640, it may take twice as long to finish your job! For most of my work, this would be the difference between 20 seconds and 40 seconds... but in your case it's the difference between 1 month or 2 months!
Don't bother looking at the Price/performance graphs because that assumes the retail price for a new CPU. You'll actually be paying a lot less on ebay. But the important thing is to compare the single-thread passmark score against what your cluster is using.
There's nothing special about ECC, except that you will want to make sure the memory you get is compatible with the motherboard. If possible, check the "Approved Memory List" (aka "QVL") from the motherboard manufacturer to be on the safe side.