Sintered runs cooler.
In the B4, I beleive the ESC would stay cool 'enough' without the fan, but if you can get a fan in there - do.
I would recommend, a 25mm fan, mounted from the side on the chassis blowing through the fins. Wired direct to 7.2v so it is blowing even before you are turned on.
6.5R is a good choice for 2wd by the way.
