Save up the extra cash and get the ASRock FATAL1TY K4/D3. It has the the best power delivery, best audio, and best overclocking capability of any of the Skylake DDR3 boards. The ones under $80 really suck, lol.
There are cheaper boards but for example this AsRock one has 40% of its owners given it an extremely unfavorable 1 egg rating .. the Giga above gets just 16%
You can overclock the i5-6400. A lot of OC novices are having a hard time learning (or re-learning) how to base clock and take manual control of memory timings (they limit overclock when left in auto). You will also want at least two frequency tiers up from baseline RAM for headroom when adjusting that base clock If your DDR3 is high end (like 2133MT/s or higher) it will be fine, otherwise the minimum DDR4 I would get would be 2666MT/s. <looking at you red jaron>
I personally went with the i3-6100 and K4/D3 and just used my high end DDR3 that I already had. If I had to start from scratch I'd probably get this RAM since it has descent timings, cheap, and enough speed to not be the limiting factor when base clocking.