I am comissioned to set up and manage a team to develop a custom software
component for a one-off event. My client is in talks with investors for
securing prefinancing, and so he asked me to write a business plan for my part.
In such a scenario, how do we increase chances that all three parties get what
they want? Should I better found a Ltd.? Needs by party
Especially unclear to me is how to give me and my team access to budget.
The real question is who will own the software at the end.
Will you own the software? In this case, the client becomes a customer, and pays you for a license for the software for their case. You can get a budget by setting up some sort of contingent sale, where the customer promises in writing to pay you for software which allows them to satisfy the basic needs they lay out in the contract. It would be normal to get an advance of, e.g., 30% of the total contract, with the rest payable on delivery. Note: you may (and don't have to) end up being paid less than your total cost to develop the software, and will develop it anyway, because you own something you can resell to other customers as well. Will the client own the software? In this case, you are working for hire, and you become an employee. In this case, the client is getting all the benefit of the software, and to reduce your risk, you should be paid like an employee: fixed contract with regular payments as you go, whether the product works as expected/hoped or not.
Is the client trying to have their cake and eat it too? Do they want to have their money be an investment in your company, where they get the software, and you split shares in a new company that owns the software? In this case, try to separate things between a contract of what they should pay for the software in a commercial transaction: which they pay you as a sale, with the extra cash they give you being an investment in the company, at a 'standard valuation' for an early stage company (this is negotiable).
The business plan is really none of their business unless they're picking this last option, but you may share it to be nice, as part of building the relationship.