Hi, I have used the M42 for trackdays for about 8 years now. Both as turbocharged and N/A. Here are my opinions about your questions.
Oil: I've been using a good quality, fully syntetic 5W-50 oil. No bearing failures yet. Don't have much experience of the Redline 20w50 but the cold viscosity sounds a bit thick? I also would not go over 50 warm visco as the oil then probably would get too hot.
Oil baffle: I'd recommend using one, especially if you have lots of grip and are fast

Oil level: I've been filling just to the top.
An oil cooler is nice, especially if you are using a thicker oil. I've installed a standard E30 325 oil cooler and my oil temp during trackdays does not exceed ~100-105 degrees C even with a turbo.
I have no experience with failing radiators, maybe I've been lucky and/or longms unlucky?