This.

If you don't have a fan right away, your probably not going to kill it, unless your gearing is way off. But I would get one. In my case I had to find a spot to make it fit the way I wanted on the chassis, which meant cutting a bit of the end of a side brace off, and then I had to modify the body somewhat. I got away with leaving the body alone for awhile, but it seemed to get more cumbersome to have to place the body perfectly so it wouldn't rub, so I ended up just trimming a spot away.

Look at the guys that win week in, week out. See what they are doing, how they are mounting and use it as a guide. Its not always about being able to "gear up" or charge batteries for max performance. Most of the times those guys will be good regardless of the car. They may just want to save the equipment.

Fan. The cooler the electronics the better. 120 is better than 160 (even though 160 will not burn the motor).

Better to run a fan.
It can allow you to run things harder if you so desire, but it can also extend the lifespan of the parts.
In addition to reducing winding temps (and to some extent, rotor/core temps), it also reduces the temperature at the bearings.
Bearings have an rpm:temp curve, where the allowable rpm is inversely proportional to temperature.
Given the high temperatures and high rpms on our motors, we run close to the allowable peak.

Also, once a motor has been overheated a few times, efficiency is permanently reduced. I've heard a few here say this, and that's my experience also.
My only caution would be to not gear right on the edge if you do use a fan (especially in offroad), as occasionally a rock or stick stops the fan which can then let the motor overheat if you don't have temperature telemetry.