The agreement you have should reflect how well you know the person, how well you trust them, and what the worst case scenario is. I'd suggest you almost certainly want lawyers involved in this part as they can give you a fairly boilerplate agreement that's tailored around the 4 or 5 key questions that you'll need to think really hard about.

Bear in mind that in a partnership, rather than a limited company, you have full personal responsibility for all debts (and not just in the normal course of business: you also need to consider someone suing you, such as disgruntled client). A simple partnership, without an explicit agreement to the contrary, will also include joint and several liability. In other words if one partner were to do something incredibly stupid, all the partners would be liable for up to the full amount owed.

There's no reason why that sort of set-up _can't_ work: there are numerous businesses that run like that quite happily, but you should be sure that you're comfortable with the risks.