Seller-financed note buyers are looking for a 9% - 14% return.
If your property is residential single-family owner-occupied in a good area, the buyer has good credit, made a good down payment, and twelve or more on-time payments have been made, you'll be in the lower range.
Note buyers, just like lenders, won't go above a certain loan-to-value (LTV) ratio. With discounted notes, it will be "Investment to Value" (ITV) rather than LTV, but the idea is similar. If a single-family owner-occupied property is worth $400,000 - I wouldn't invest more than $280,000 (70%) in a note secured by the property.
ITV takes priority over desired yield - so even if the note discounts to $350,000 based on desired yield, the investor still won't go beyond their maximum ITV.
With newer notes especially, partials reduce the discount.
I'm not a fan of balloons for seller financing - especially shorter than 10 years. If your rate is higher than institutional rates - they'll be incentivized to refinance when possible.
Structure depends on what payment the buyer and the property can afford.
Look at area rents. Base the payment amount so you're in that range. Find a buyer with at least 10% to put down and can afford that payment.
I'm fairly sure you can structure to sell a partial after a month or two of payments that would pay off the underlying $215K note.