Dont buy a used trimmer. If your stuck on $200 than the Kawi. Its not a true commercial trimmer, but I have one and its pretty close.
I used to be the guy that laughed at a trimmer if it was under $350, but honestly, with how we run them, I cannot seem to get more than two years out of a commercial trimmer now matter how much it costs. Had high end Sthils, Echos, Tanakas, and now kawi's. Same thing, stick edger or trimmer, almost two years on the dot..... trashed. We use them all day every day though.
Now I reserve the high end trimmers for brush work, and the cheap $200 trimmers seem to work well for lawns that are already well manicured.
I'm going to try Husqvarna next. Bought their $200 stick edger, and it has just as much power and tuns just as well as any $400 edger I have had. Their bottom end commercial trimmer is $250, but if it runs anywhere near how their cheap stick edger does, I will be happy.