History: resourcing

You can either get internal or external resources. Identify the resources and/or internal training. If you involve contractors, you might want them to quote the work beforehand and take account of this in your own quote, see managing contractor. In any case you need to be sure of the availability of the people you want to involve.

If you're going to work with contractors that you don't really know, and you get a strong deadline from the client, there might be a risk of over time.
Because contractors don't have a strong obligation to provide the work or a good quality work especially if they are in a different country. And if you need to involve another person in a middle of a project, it will take time.
To prevent this problem you can involve more than one contractor on the project and split the task and so if one fail the other might be able to undertake the project as it will already know it.
Another more expensive solution is to involve two contractors for the same development and so charge the client more.