Buy i5-2400 and as motherboard: Intel DH67CL.
Best combo for you.
There is no real world difference in performance between i5-2400 and i5-2500K.
Now, let me be very clear to the following points:
1. i5-2500K is unlocked processor, that means you can overclock it with particular chipset. (i.e. P67 and Z68 and Z77 which is released yesterday)
2. You are on low budget, so you won't get quality any motherboard based on any of those chipset.
3. You will need a after market cooler like Hyper212 EVO which is priced @2.1K even if you don't overclock i5-2500K. With i5-2400, you won't need any.
4. No game required overclocking.
5. You can overclock i5-2400, just by increasing BLK frequency, and it can be done upto 3.9GHz, which is enough to put out your overclocking need. (You will need a aftermarket cooler for that)
6. A good PSU like Corsair CX430V2 is the least requirement for overclocking. CoolerMaster Extreme series won't do, believe me.
And at last, please ignore the above post made by fellow poster 'rcraktimc'. It is his post, which made me post this long post with some info for you which may prevent you from being confused by the mentioned post.
Even if you use IGP, there is no real world difference between HD2000 & HD3000. (both are crap) Unless you want to see slideshow of frames, use a mid rage graphics card from AMD. (remember, nvidia doesn't have any good mid range graphics card which is available at a very good price)
I would suggest you to get any good HD6850, which can be found @~8K.
So, to sum everything up, your config should be:
Intel Core i5-2400 @10K
Intel DH67CL @5.1K
<any brand> HD6850 @~8.5K
I don't know which brand of 6850 is better. Wait for the reply from the guys who have HD6850.