For a quality trimmer that can handle brush cutting your going to have to up your price limit a little. The best Stihl trimmer with that kind of capability at the lowest price would be the FS90R at around $300. It has the 4-mix engine, which is very torque-y compared to a conventional two stroke, it has a solid steel drive shaft, and a gear box that can handle attachments or blades. I have this same model and have been using it for the last few years. Putting it through it's paces in grass trimming and brush cutting, it has worked flawlessly.
If you want heavy brush cutting though, I would step up to a FS250R, but there closer to $400+, but well worth it.