Make sure you also have a chat with your friend about the difficult stuff: what happens if one of you doesn't pull their weight; when is it OK to accept the bus

Make sure you also have a chat with your friend about the difficult stuff: what happens if one of you doesn't pull their weight; when is it OK to accept the business isn't working; at what price will you sell your stake; and so on. If you have this worked out in advance, there's less of a chance of destroying your friendship.