" so it'll be tough to build an internal team quickly enough "
This smells of failure. Contractors aren't going to get up to speed any faster than internal resources (sans technology specifics like expertise in a language). Our management tried the same thing: hire contractors for a short term (less than 3 months), hurry up scenario. Except it took a month to interview and get the contractors on site. Much of the 3 months of contractors time was spent to get their environments setup, work with IT to configure permissions and the contractors themselves to learn the complex product enough to contribute. Not to mention the loss of focus of the internal team assisting the the contractors.
I would spend more effort coming up with a realistic plan that has a chance at success rather than trying to meet a date that is not going to be met. Build a plan that includes a mix of internal an external resources. I would include time to hire contractors (remembering that background checks take time) plus all of the other activities that will consume time away from producing the finished product.