I've never seen one that will automatically dispense new string. Mine have always been "bump" feed. Rev it up, and bump it on the grass, and it feeds out more line.
I've never used the plastic blades. I imagine they would be good for heavier weeds and such, but I have no experience with them.
I just bought a Stihl chainsaw for "homeowner" use, and it is a very high quality tool. Over here, they are considered to be one of the better/best brands. Echo is another brand that is supposed to be equally well made.