With a lack of plasma disruptions and current-driven instabilities, stellarators are potentially an attractive option for a fusion power plant. Previous system studies have been performed to optimise a HELIAS (HELIcal-axis Advanced Stellarator) 5-B power plant using PROCESS, however these have been based around a single design point. In reality there is a lot of uncertainty extrapolating from present day devices and understanding. In this paper we study how this will affect the design by identifying eight uncertainty distributions on the input. We then perform parameter studies and Monte-Carlo based analysis to look at the impact on fusion power and divertor heat load. We find that the two uncertainties that have the largest impact on the fusion power are the helium primary coolant mechanical pumping power and the energy multiplication in the blanket and shield. Eighty-three per cent of our solutions are within a tolerable divertor heat load, however this is additionally influenced by the tungsten impurity levels. In order to stay below the density cutoff limit for Electron Cyclotron Resonance Heating, the confinement time needs to be enhanced relative to the ISS04 scaling relation to produce acceptable performance. By identifying the highest impact design parameters, we are able to highlight the areas that need to be prioritised for additional study.