Hi,

I'll be making some CF wheel arches for my Caterham shortly and would like your recommendations as to a durable (lots of travel will hit them) resin. The arches will remain unpainted and the process I will use is vacuum infusion.

Ta in advance :-)

I highly doubt, that a thin Kevlar layer will give the laminate any protection. The problem is load distribution. A sharp stone transfers a force on a very small area. A thin Kevlar layer will not change that. If the local force is too high for your layup, it will crack. Kevlar holds everything together, if it fails, and has a better abrasion resistance, but it will not prevent any damage (except abrasion). At least not better than any other material. A thin rubber sheet is a different story, as it distributes the force.
That's  the first thing I wanted to say. The other is: I've done quite a few sets of carbon wheel arches for seven like cars. I never used any protective layer, and never had any complaints. Some customers took them to the racetrack including some distance at high speeds through the gravel trap.
The standard arches are made of chopped mat, which is exceptionally weak against chipping and cracking.