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To achieve a high system availability at minimal costs, relevant decisions include the choice of preventive maintenance frequency, spare part inventory levels and spare part repair capacity. We develop heuristics for the joint optimisation of these variables for (a) a single k-out-of-N system under condition-based maintenance and (b) an installed base of multiple identical k-out-of-N systems under block replacement. We show that a straightforward extension of the METRIC method for spare part inventory optimisation yields inferior results, because both the availability and costs are not necessarily monotonous functions of the decision variables. We develop an adjusted marginal analysis and show that it performs considerably better in numerical experiments.