Better approach for maximizing Cooling Fan Life Expectancy
This is just my two cents but it would seem that you wouldn't have to worry about heating up the fan coil so much if you had the entire casing using fans in a positive pressure system. I'm assuming that the fan(s) you intend to use are brushless and you are putting a filter in front of the fan(s). This would make having to worry about normal dust as well as carbon dust from the brushes a non factor.

As for running the motor either at a higher set point or controlling the fan speed I would opt for the latter. Strangely enough I would as much as I could try to keep the fan moving when possible if you are going for longevity. Letting anti-friction bearings sit is possibly one of the worst things you can do for them. After a amount of time they will get flat spots and make metal to metal contact as their lack of movement would mean no wedge of lubricant to ride upon. That would be the sound you hear on old computer fans if their bearings are going out.

It might seem counter intuitive, but you could always have a way to plug the fan motor(reverse it's direction) if you have a filter built in ahead of it. This would allow it to automatically clean the filter a bit if the rpm of the fan motor dipped below a certain amount for the voltage being supplied to it.

This all depends though, I would look at the MTBF of the fan(s) in question before I made any decisions. Hope this is helpful. -J.G.