The Multi Element Telescope for Imaging and Spectroscopy (METIS) is the coronagraph selected for the Solar Orbiter payload, adopted in October 2011 by ESA for the following Implementation Phase. The instrument design has been conceived by a team composed by several research institutes with the aim to perform both VIS and EUV narrow-band imaging and spectroscopy of the solar corona. METIS, owing to its multi-wavelength capability, will address some of the major open issues in understanding the physical processes in the corona and the solar wind origin and properties, exploiting the unique opportunities offered by the SO mission profile. The METIS Processing and Power Unit (MPPU) is the Instrument's power supply and on-board data handling modular electronics, designed to address all the scientific requirements of the METIS Coronagraph. MPPU manages data and command flows, the timing and power distribution networks and its architecture reflects several trade-off solutions with respect to the allocated resources in order to reduce any possible electronics single-point failure. This paper reports on the selected HW and SW architectures adopted after the Preliminary Design Review (PDR), performed by ESA in early 2012.