I want to kill my lawn and install low water plants. Can I use roundup for a few applications, mow the dead grass as short as possible, install plants, add drippers, and use bark over the old lawn? I don't want to pay the expense to have the old lawn physically removed.

Your plan is very sound. One application should take care of all the grasses, spot treatments of stubborn broadleaves for complete control may be needed. Not disturbing the soil will also benefit you in that very few weed seeds will be prompted to sprout and grow. While you are planting low water usage plants, do not forget that they too require a good amount of water to establish themselves before they become self sufficient. Also do not get mulch up into the crowns of these plants. Most perennials cannot grow up through mulch very well in the spring.