I would give them the general idea of what you are looking for in terms of a final product. From there, you should discuss price and budget. Its very important that terms and conditions are figured out immediately between you and the design firm.
The important thing, and this will depend on the size of the product, is agreed upon delivery stages and qualifying points for payment. Generally when I prototype something, I expect something like roughly 10% for the basic designs and drawings. 35% for the mechanical deliveries. 25% for the electrical deliveries, and then the rest of payment for final site delivery and payments. Failure of me delivering these stages on time result in a penalty on my payout.
However, design changes, failure to secure delivery conditions (import regs/inspection standards/just whatever the fuck that is not covered in contract, the end users eat the cost as shown in the terms and conditions).
This is the general path when I proto-type shit like below.
This is not my image or product, but it is the general size of the electrical and mechanical systems design and build.
I'm not sure the size of what you are trying to get created. But no matter the size, the most important thing is the terms and conditions, because this is what protects both parties.
