I would advise against cutting down small trees, or anything tougher than grass. The stress on the drive shaft and clutch is just too much and your strimmer will fail prematurely. Weed eaters/ strimmers are tools best designed to cut grass and weeds, stuff without tough bark and solid trunks. I'd rather use a chainsaw for a tree, or an axe for smaller plants that need to be removed.
cheap as hell in eBay. 13 dollars delivered in australia. just make sure you check the bolt every time you use it, because if that bad boy comes off,,,,, someone is in deep shit