The Extreme Polarimeter, ExPo, is being developed for the detailed study of circumstellar disks and exoplanet characterization at the 4.2-m William Herschel Telescope at La Palma. This imaging polarimeter is designed to measure linear polarization at the 10-5 level around bright stars at distances outward of about 0.5 arcsec. We will discuss the design options for this instrument as well as the advantages and disadvantages of specific components such as the Atmospheric Dispersion Compensator, the polarising beamsplitting element, the coronograph mask, and the polarisation modulator based on either ferroelectric or nematic liquid crystals. The merits and disadvantages of each of these will be discussed along with the impact of a certain choice on the overall instrument performance. Finally, an analysis of several off-the-shelf scientific cameras for the actual image recording will be presented.