What are you planning to do? Video games - rendering - video editing etc. Are you going to buy aftermarket cpu cooler?
What is your budget?
If you are planning to play video games, you should choose 3600 instead of 2700 (prices should be close) as 3600 has better single core performance.
If you are not looking for top FPS numbers, and on tight budget, and want to be future ready, 2700 or 2700x would be the best option.
I would eliminate 2600 unless you really have tight budget, as 6 core will not be future ready as 8 core, and single core performance difference between 2600 and 3600 is huge.
Mobo: Definitely get the B450. You don't need x470 at all unless you do extreme overclocking.
I would recommend Asus w/ b450 platform. I bought 2700x and MSI b450 mini itx for my wife's gaming pc, and the motherboard got bricked after downloading the latest chipset. It always caused problems, such as turning itself off when restarting, blue screen etc. I cleared CMOS, tried different versions of BIOS but wasn't able to make it fully stable. After spending days and nights trying to figure out the problem, I finally gave up and returned MSI mobo and got Asus TUF B450 mini itx board. Since then, there is no problem. I am definitely not looking back and I will avoid MSI branded products like a plague. It is not just me, it is a common problem especially among AMD users, MSI's software support SUCKS.
Although Asus is not perfect on the Intel side. Their z390 motherboards are actually 4 phase VRM but they advertised it as a 8 phase VRM, which is NOT sufficient for power hungry 9900k cpu if you overclock it. This is the main reason (misleading advertisement) why I went with Gigabyte z390 which has 12 phase vrm with my 9700k. I am not super happy with Gigabyte bios though. It's been problem-free but BIOS is clunky for overclocking. As far as I know, their bios is also not the best (not as user friendly as Asus) with AMD platform.