Service-oriented computing (SOC) suggests that many open, network-accessible services will be available over the Internet for organizations to incorporate into their own processes. Developing new software systems by composing an organization's local services and externally-available web services is conceptually different from system development supported by traditional software engineering lifecycles. Consumer organizations typically have no control over the quality and/or consistency of the external services that they incorporate, thus top-down software development lifecycles are impractical. Software architects and designers will require agile, lightweight processes to evaluate tradeoffs in system design based on the "estimated" responsiveness of external services coupled with known performance of local services. We introduce a model-driven software engineering approach for designing systems (i.e. workflows of web services) under these circumstances and a corresponding simulation-based evaluation tool.