The audio sizes work out to be this size in pcm/ac3:
2.0 mix 1.25 GB
5.1 DTS 4.1 GB
5.1 AC3 around 350 MB
That's around 5.5 gigabytes for audio.
The menus,etc. should be under 1gb
This should leave at least 18gb available for the x264 encode, so it seems like 720p should be totally ok. I guess re-encoding the 5.1 ac3 to pcm would be a bad idea.
This is for esb97se but should be comparable for the other 2 films.