I'd be wary of going with a customer/inventor, you might end up building a solution too tailored for them that you can't resell easily; and there are conflicts

I'd be wary of going with a customer/inventor, you might end up building a solution too tailored for them that you can't resell easily; and there are conflicts of interest for them. Given the growth you're having organically on just two people and not that much time, I'd try to keep on your own a little bit more rather than searching for investors. That will take time, and hiring more people will also take even more time. Maybe it's worth it to put that time into getting new clients, knowing the market (i.e., what's your upper limit, what companies you can't reach, etc) and maybe growing revenue so when you do take investors and hire more people you're closer to breaking even.