It depends on the ambient air temperature in the location in which the brooder will be. The ambient air temp in the spare bathroom my brooder is located is about 85-87 degrees with the AC vent blocked and the 250 watt heat bulb on. The chicks like the warmth provided, but move around. Sometimes they're close to the area that is warmed by the light, then they relocate to a cooler area. It appears to me that they like varying their environmental temperature, most seem to do this.
The plastic socket won't take a high power bulb, the rating should be stamped into the fitting someplace, 100 watts sounds about right. However, the cost of the high heat tolerant light fixture, at $9.00, is really too low to compromise in my opinion. I think the red heat bulb is about $8.00 more dollars, but a regular 250 watt white flood light bulb will work, I think they're cheaper.
I have one end of the brooder under the light, with the remaining two-thirds outside that.