The plastic is just a moisture barrier. Not absolutely necessary under a garage but will help a little if you are going to heat it. Also helps with radon gas. Put it on your subgrade (sand/gravel) and under your remesh. Pour the cement right on top. Ask your contractor.
Curb, gutter, and sidewalk may not be that much more. Talk to your contractor. He will probably form up the curb and gutter area and adjust the slump on the concrete so he can hand form the curbs and gutter which is not a problem for someone experienced at it. Good luck.