So, you should be good with the 8700k then, or with a Ryzen 3600, whichever is less expensive and is readily available. They have pretty similar performance but the R5 3600 tends to be much cheaper in most markets. Same number of cores and hyperthreads. The Ryzen actually has faster single core and multicore performance than the 8700k at it's stock configuration.
The main issue with most microATX boards, compared to the standard ATX versions, is that they often lack some things. The don't usually have AS good of power delivery components, but sometimes they do if it is a high end board. They tend to usually have fewer fan headers, fewer SATA headers, some boards might only have two DIMM slots instead of four. Fewer PCI slots for add in cards. They are just generally pared down versions of the same, larger model. Some of them, at the upper end of the scale, are actually very high end but it depends on the model. Obviously, to some degree at least, you get what you pay for when it comes to motherboards but at some point that stops being necessarily true and starts changing to you are paying for strictly high end features like water cooling integration, gaming or overclocking features that might not even be useful to many users or unnecessary high end features like 10GBe networking that very few people can even use at all.
For most people these days, any mid tier board in the 125-200 dollar range is plenty good and has everything you could really want or need unless you are a higher end user with very specific overclocking or productivity requirements.
For gamers, any of the recent generation 8 core or 6/6 or higher model CPUs, 16GB or higher of memory, an SSD of some kind and at least a moderately capable graphics card along with a high quality power supply and sufficient cooling, is pretty much optimal.
It really does become a matter of "what can you afford" and what is available to you, when it comes to "what should I get". I tend to recommend targeting hardware that could reasonably remain viable for about five years, because beyond that some hardware or other is likely to fail and not be worth trying to replace once it does, anyhow.