If I use a normal build of ffmpeg how take advantage of video acceleration for scale?
The answer to your second question is yes. Change -c:v libx264 to -c:v h264_nvenc.  I use those builds as well.  If you use the option -encoders you can see a list of included video and audio codecs. 
This is a good resource for encoding with FFMPEG.

See the warning on this question if you try this