Minimal DSP for video transcoding

Hello. I am new to DSP, even more - i am not even in DSP theme at the
moment.
Problem is - i need to transcode video from 12mbps MPEG2 to any other
format, that will need less bps. In realtime.
For example i can downscale the video (from FullHD to 640x360), or i can
transcode to H.264
My main ARM-based CPU has insufficient processing power to do this in
realtime. So i need external IC, that will do the work.
MPEG IC's are now outdated and most of them are EOL. So i need FPGA (that
have the price far over budget) or DSP.
Here comes the question - what is the minimal DSP to transcode/downscale
the video stream?
I understand, that the answer depends on quality of code and exact
pipeline, but in theory - what is the minimum?

	 

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