Are they a high net worth individual?

If not, there are additional legal issues you have to contend with. Various "protect the little old ladies from being taken for a ride" laws make it more difficult for individuals who aren't worth millions to participate in private investing. At least, that's the case with corporations. With an LLC and proprietorship, I guess it's more like a personal loan and all of the legal stuff that surrounds personal loans would apply. Presumably you plan to, or already incorporated. As you've noted, spending a couple of hours with a lawyer is probably a wise investment. Just a phone call to a startup-focused lawyer will get you a few minutes of free advice on what you need a lawyer for (the better a lawyer is, the more likely they are to try to avoid doing extraneous work for you--they may charge twice as much, but they'll put in significantly less billable hours). Surprisingly, perhaps, raising bigger money from angels (who are high net worth individuals) can be less legally complicated, and the boiler-plate stuff that YC provides will probably do the trick.