You don't need to be very worried.
Keyboards and mice are insulated plastic. Static shocks to the grounded PC case can in theory be an issue, but in practice, not really. The biggest danger is static shock while working on components inside the PC- there's no protection there, and as little as a few tenths of a volt can be fatal to a modern microchip. They won't be doing much of that, though, will they?
Just make sure everything is properly and thoroughly grounded and installed robustly enough to take decades of abuse without becoming a safety hazard. Dry sandy soil has terrible conductivity, so you may have to drive supplemental grounding electrodes to get reasonable resistance to ground.
Also, if it really is that dry, you might want to think about installing a swamp cooler instead of an AC unit- would be cheaper to operate and woul humidify the air, too.
