The 5000 series processors are Haswell-E, the 6000 series is Bradwell-E, BW-E is an update to HW-E , BW-E will be the last processor for this chipset which is to be replaced later this year with Skylake-E and a new chipset and socket. The K processors have an un-locked multiplier that allows overclocking
HW-E typically can overclock higher than BW-E but BW-E out performs HW-E at same speed, and there is no reason to purchase the later at this time. 6950X is the bad boy on the block, the most powerfull desktop CPU available at this time.
As for memory, power supply etc., don't cheap out, get top quality components that are compatible, 128GB memory kits are costly and more susceptible to issues so make this purchase carefully, there is a guide on these forums for setting up a 128GB memory kit and several users that use these kits, search the forums for related posts