@talentryto- I think that a plastic trowel will work fine as long as you are careful not to break it. Though you definitely will have to be more gentle with it than you would a metal trowel, it will still get the job done.
However, if you have a project to do that requires a lot of mortar and bricks, you might just want to invest in a metal trowel. Plastic trowels do have the tendency to break after a lot of wear and tear. In the long run, you could possibly save money if you get a trowel that will hold up after using it to do a lot of work.