Started my business, got big client, need help.

This past year I've started my own software development company as my primary job is very seasonal and finally we've got a BIG client – we usually deal with easy websites but now we have a large software project. A job like this in my primary workplace would be a walk in the park but now branching off (with only one other employee) and need to figure out legal documents / agreements and planning – stuff that was all done for us.

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The project requires knowledge with hardware, which we don't know how to interface with, but would like get someone to look at it as the programming side is very easy and right in my area of expertise. Would it be possible to form a contract that isn't going to cause a legal dispute if my business cannot figure out the hardware interface?

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I've been looking at creating a 'Milestone' type contract where the payment is divided into parts, upon completion, a certain percentage of the total payment is received.

EDIT: Upon writing this, I realize I should've gone to a law sub-reddit but maybe someone's past experiences could help me out, thanks!