The proposal seems overly complicated. It also front-end loads the terms against you. That is, if times are tough the first few years then you'd be paying the minimum. Then, if things are rolling better you'd be unable to catch-up due to compounding interest.
In your 6% : 10% proposal, your benefit of not offering a fixed 10% is that it gives you an outlet if things are bad. However, the difference between 6% and 10% isn't all that much. If you've got a great deal, then go with 8% fixed. I wouldn't offer more than that just because it is such a struggle as an individual investor to make much more than 8%.
I'd probably start-off proposing a fully amortized loan at 6% for whatever term. By fully amortizing the loan, they'll receive more interest during the first part of the loan. If you choose to pay it off early then their yield will be greater than 6%.
The loan is simple if the only variables are interest rate and term. If you need an outlest for generosity (or guilt), once or twice a year, send the lender a thank you package of fruit, candy, flowers, gift card to restaurant, iPad, etc.
Good luck in Iowa, I'm from Ames.