I have been working as a Business Entrepreneur and have been working with many of the customers from Europe, North America. While creating a White Label solution, the most important part for the owner is to manage the security for the product as well manage the Quality to be delivered within the expected timeline and budget. While choosing a company, the NDA's would work but with them you can also sign a Proposal, SLA(Service Level Agreement) which helps to bind the contract and source code as your own property. As when it comes to offshore development, my suggestions is to choose a company which has been doing this services since more than 5 years now and have been a part of good success stories whose stories are kept confidential. So with the documents, a formal trust would help you create the platform as your own property with the expected quality.
You can consult your Legal adviser in the matter and hope they would align the strategy.