Here is a link to the processors supported by your motherboard.
The i7-4790K is the strongest on the list.
It is supported by F9 bios and later.
CPU-Z will tell you your current bios level.
DO NOT update without a good cause; a failed bios flasn can cause all sorts of problems.
Broadwell had much better integrated graphics, but no more performance. Since it is not supported, it is a moot point for you.
Kaby lake will work on current skylake lga1151 motherboards. The requirement for low voltage ddr3 or ddr4 will still be there.
Then, also, Intel does not reduce prices for old gen products. They maintain full list for replacement buyers.
On the used market, you will get fair price/performance.
Since kaby lake is not expected to be much faster and not cheaper, you will be better off with a i7-4790K.
One place for a real deal is microcenter if you live nearby; they will sell you a i7-4790K for $270
http://www.microcenter.com/product/434176/Core_i7-4790K_40GHz_LGA_1150_Boxed_Processor?rf=Search+Results+Feature