I am not big at all on startups spending their limited funds on "standard" legal agreements if they have a "standard" business. That said, THIS is not one of those agreements to try and DIY. The ramifications can be immense, and this is a classic one to be litigated and parsed in the extreme. Just like in marriages, business partners drift apart, and it can be shocking what seemingly small sums of money will be fought over tooth and nail. You don't need to break the bank on this (find a good local startup attorney and they should have a form you can start with that has all the generally important clauses included), but doing this particular agreement without an attorney is a very bad idea, imho.