1 : Absolutely. Running extremely high voltages and frequencies can shorten processor life exponentially. However running a mild overclock usually won't hurt the CPU life at all.
2 : Intel K series processors have unlocked multipliers. i5-2500k and i5-3570k are good to look at in this area. If you're looking to overclock and you buy a locked non-k intel chip... well you get what you deserve.
3 : There's so many out there today. I recommend the ultra cheap yet very good performance Coolermaster Hyper 212 Plus. It can be had for cheap and performs extremely well.
4 : The answer is NO. On the last two generations of the i-series processors, the K-Series chips have eliminated any front side bus overclocking from being needed to overclock just the GPU. You CAN overclock the ram or GPU, but it is not a necessity.
Honestly, if you went with this...
Z77 motherboard (decent quality/OC ability)
Intel i5-3570k 3.4Ghz CPU
Hyper 212 Plus (Don't forget to LEARN how to properly apply thermal paste!)
8Gb to 16Gb of DDR 1600 or 1866mhz ram
Decent GPU like a GTX670
You'd have a darn good gaming rig. Also overclocking that chip to a modest 4.0Ghz for a 600mhz overclock can usually be done with no voltage changes. Also it probably, with the Hyper212 will run colder than the stock heatsink would run at stock speeds. As for the lifespan of the chip, I can say that a modest 600Mhz overclock on it probably won't hurt the lifespan much if any. Now pouring heavy amounts of voltage in the chip can cut the life in half if not worse.
I hope this helps out! Good luck and happy overclocking!