I am a Pentium man so these AMD CPUs are a bit of a mystery to me, by all means if you want to splash out then go for the 8 core, more cores always better IMO.
B450/X570 Different Chipsets, basically the X570 is better. It manages all the flow of data in the motherboard. If you have a shit Chipset with a good CPU you may not get the performance you expect. The B450 boards appear to show up in budget builds with R5 2400G CPUs. And an older one may not even utilise the CPU properly, I recall I had to make sure I got a specific chipset in mines or I would be unable to OC.
If you dont plan to OC you should be ok with the stock fan, but personally I prefer having an aftermarket one, you can get cheaper ones than i linked which are still better than a lot of the stock ones simply because they at least direct airflow out the case rather into a side of it (like stock ones normally do) which helps keep everything cooler.
OC itself, upto you I guess, I like to squeeze that little extra out mines and I have a dedicated air cooling case (I have 3x120mm Fans that blow air from the bottom up, my MOBO is rotated so the 'back' is the top of my case, i.e. warm air flows up.) and a massive CPU cooler to OC and have OC'd my last 3 CPUs and never had a problem. (tbf the first one was a pre-built OC).
Its safer than it used to be, usually the MOBOs have software to do it for you and then if you do some stress testing and it passes you should be good.