This is going to be my new project.
A G-Tech sweeper will form the basis of the robot. It is a 7.2v rotary brush with dust collector. The rough plan is to:
Pull it apart - where all good projects start.
Figure out a drive train - which will run off 6v motors.
Design a sensors array/grid, which won't interfere with the sweeper.
Bolt on an MPU.
Get it to clean the house!