We bough a Swisher walk behind string trimmer for use around our farm. It is amazing, and is ofset to the left, so you can walk along a fence, or the edge of a field, and use it without having to push under the fence, and it save your arms big time. The string is very, very heavy duty, and it will actually take down small saplings if you are not careful. We use it under the edge of a pasture fence, on a hill side, around the house, and in the deep weeds that are almost waist high, and very thick. The only thing is you have to wear eye protection, but it comes with it, because it does kick back some debris. Stihl is also a top notch brand if you want the kind he is going to carry. We use the Stihl Farm Boss chain saws to cut the wood that we heat our house with, and it cuts like butter and starts every time, and the Stihl dealer is very helpful. Below is the link to the Swisher, just as an idea and its on sale for $279.
Swisher 6.75 Gross Torque 22 in. String Trimmer - 1039311 | Tractor Supply Company