I had a 5.5 in my 4wd on the MARCCA carpet, it would get turned down to about 85% & was very drive-able.

I have ran a plain old 6.5 in my 4x4 SCT & under geared it & boosted it to the hilt. I wouldn't recommend it...

All & all I'll take a touch too much & turn it down then melt out the motor from wanting more & asking too much.

Bang for the buck I'd try a Speed Passion 6.5 V3 (new $40) & dial accordingly.
If you have a boost-able ESC & are comfortable with setup, a boosted undergeared 8.5 would likely be more than enough.

As far as temperature, you can get either hot... it is a factor of duty cycle & load.
In general the 6.5 will run "hotter" than the 8.5, but if you gear &/or boost either improperly you can heat them up fast.
I have 6.5's that are 5+ years old that have been ran in Slashes outdoors & they would do 130-150 all day long & have no issue.
Burning the tires off & not having any finger control will heat up things a lot too.