There is some controversy over minimum bitrate standards for dvd video. Many people seem to think it is around 2000kbps, but I haven't actually seen it in an official standard. There is a maximum setting, but that doesn't concern you here.

Essentially mpeg2 video simply doesn't work very well at extremely low bitrates, though there are some encoders that are better than others at the low end of the spectrum. Procoder has a rep for this.

Your dvd players may not play back the video well if you set things too low. And you are liable to get jerky, stuttering video.

If you want to test things, I'd go to vbr encoding and use the settings I said before. The video will have an average rate somewhere near 3000kbps, and will have enough headroom with a max of 5000 to get good quality on the action sequences.

Make a test at these settings. Then lower the settings if you like and start making comparisons. None of this would be acceptable to me, but you need to judge these for yourself.