Power electronic converters will serve as the fundamental components of modern power systems. However, they may suffer from poorer reliability if not properly designed, consequently affecting the overall performance of power systems. Accordingly, the converter reliability should be taken into account in design and planning of Power Electronic-based Power Systems (PEPSs). Optimal decision-making in planning of PEPSs requires precise reliability modeling in converters from component up to system-level. This paper proposes model-based system-level design and maintenance strategies in PEPSs based on the reliability model of converters. This will yield a reliable and economic planning of PEPSs by proper sizing of converters, cost-effective design of converter components, identifying and strengthening the converter weakest links, as well as optimal maintenance scheduling of converters. Numerical case studies demonstrate the effectiveness of the proposed design and planning strategies for modern power systems.