Sensitive electronics, especially computers, don't like excessive dust, lint and dirt and will shorten their life if not brought in for regular servicing to clean out all the interior dust, lint and dirt.
Doing a thorough cleaning inside of an iMac is NOT something a user can do, unfortunately.
Is there anything you can do to make the environmemt where the iMac would be more dirt and dust free?
Instead of putting a very expensive iMac out into a dust filled environment where its longevity WILL be affected, have you considered purchasing a cheaper model iMac or even a Mac Mini ( you can purchase a cheaper monitor and will have to add a keyboard and mouse/trackpad) that are, also, less expensive and maybe more expendable if those models succumb to early failure being out in a farm environment.
I would never think of buying an expensive Mac and put it out into a dust and dirt filled environment, even a base model iMac.
I probably be all right with putting a Mac Mini out there with a cheap monitor to take a beating in that environment.
A mid range Mac Mini will only with cheap monitor will, probably, set you back less than $1000.
Much cheaper computer to sacrifice in that type of environment than a 27 inch screen retina iMac.
