If you decide to have some or all learning objects developed externally, choose a partner with experience in instructional design and in developing e-learning objects. For example, do not go to a website developer to develop tests or simulations.
Work with projects and provide for sufficient time for the coordination, monitoring, testing, validation, etc.
You stay the expert concerning content and you stay responsible for it.
Ask the developer to develop a small piece (prototype) first and make adjustments until it meets your expectations.
Today, compatibility with all kinds of devices (PC, tablet, …) is crucial. For example, make sure that the simulation is not developed in Flash if you also want to display it on an iPad.
Finally think of updates (content adjustments for the future). Which can you perform yourself or should you consult the developer (and pay) again?