Whether you're looking for fixed-price bids, cost-per-line, cost per hour, etc, you should specify more than you've specified.

Do you want hardware design? Do you want code in 'C' or in ASM? What specific tests must be passed before final payment is made? What terms do you expect for payment and delivery? How firmly fixed are your system requirements? In what volume do you plan to manufacture?

What is the time-frame? Which MCU do you plan to use? Have you got a preferred MCU vendor? Do you have a product category? Do you have a target product price? Do you have a target completion date? Whta other constraints do you have?

One can't really work with the information you've provided.

You may find that persons willing to work against a handwave rather than a specification will cost orders of magnitude more than those requiring a firm specification, and may take years longer, despite the fact they charge 10% per hour of what those requiring you to know and specify what you require.

It appears you've got many decisions to make and lots of work to do before you engage a consultant.

RE