Tbh I don't recommend X99 to everyone. It is really dependive from your usage. If you just play video games go for a Z170 platform with a 6700k. LGA 1151 supports DDR3 and DDR4, so you have to watch out which motherboard you buy. Z170 boards are much cheaper than X99 boards, you can save about 100$ and put them in your GPU. LinusTechTips made a really good video about this topic, I'll link it here for you.

The conclusion would be the following :
5820k = gaming, streaming, video editing, multi GPU (more PCIe lanes)
6700k = a little stronger for gaming and less power consumption, but slower in multithread operations.

To your overclocking thing : every modern Intel CPU should be able to hit 4,5Ghz easily, just buy a 20-30$ CPU fan (beQuiet! Pure Rock, Arctic Freezer 13, Coolermaster Hyper T4, EKL Alpenföhn Brocken, EKL Ben Nevis).