@Clayton Hepler, I would stay away from the partnership. It can get pretty messy and costly to set up (everyone needs to agree to the operating agreement that your attorney drafts).
I would just offer up a mortgage & note to them. Here's a handful of ways to structure the mortgage & note...
1. Offer them an attractive interest rate.
2. Offer them a flat percentage of the profit as their profit. This one can be a little tricky tax wise. The IRS may want to charge this to them as ordinary income as opposed to interest income.
3. Offer them a minimum amount of interest you'll pay. This one is great if you think you're going to sell fast. The faster you sell, the higher their yield goes.
4. Offer them a fixed interest rate plus a performance bonus that is based on the profit (i.e. 25% of the net profit).
One of the nice things about dealing with private individuals and not HML's is that you can get creative. The correct answer is... Whatever your investor wants from the deal. When we are on-boarding a new investor for our properties, we ask them what would make them excited to be in the deal. Or, what interest rate would make them feel comfortable and happy. In other words, let them determine the terms. It's great that you want to be generous. But, if you offer too high of a rate of return to them, you may scare them off. If they pick a low number, it's better for you in case anything goes wrong. When it goes well, there is nothing that says you can't just pay them extra money as a way of saying thank you. Love checks can go a long way in the loyalty department.
Good luck!