A couple of years ago I bought a Swisher Postmaster to pull behind my garden tractor and keep the weeds and grass out of my board and hi-tensil wire fences.
I paid about $500 for the Postmaster through Northern Tool. It works very well, but has a tendency to slip downhill on sloped ground and once in a while the unit gets hung up on a square post -- usually the result of operator error while consuming a cold drink while driving the garden tractor.
My neighbor hires a crew of several kids who spend all day running weed whackers along his fences. Meanwhile, I drink a cool beverage as I ride along keeping an eye on the cutter head. On my place, I spend about an hour or two every couple of weeks keeping the fence lines trimmed. Not hard at all and the heavy nylon cords are easy to change.
I've made a couple of field modifications to the Postmaster and sent the ideas to Swisher. Sometimes I get lazy and let the grass get too tall and then I have to use my 6HP DR trimmer to go along and clean up around the posts. Not fun on a hot day, but my own fault.
Hope this helps.
Ed