I know full well that appreciation is simply a bonus, but I do want to account for it. 

The initial renting process typically requires a few months payment up front: 1-2 months deposit, 1 month for the rental agent, 1 month 'key money' (think of it as bonus payment for the landlord).

He wants a bonus on cost savings on expenses if he comes under budget (which would give him the incentive to estimate high on expense estimates), and does not want the cost of his labor to be included in the expenses to determine net profit.  

(Another possible bonus - they have a VERY valuable property in our downtown core where their business was located -- it's listed with a commercial firm now - but I'd guess their listing agreement should expire soon - I could possibly see if something could be worked out for it as well -- however I'd need outside partners)Maybe I'm looking for some attaboys or simply encouragement - this would be my first owner finance deal if I can pull it off.  

The typical turnkey providers normally seen around the REI realm are ones buying at a wholesale price, executing rehab, filling with tenants, and selling it at the new appraised price with the added bonus of it being tenant occupied for easy rental income for an investor. - Turnkey property from my experiences mean a property that is move-in ready.

Another bonus is another set of eyes.

You need to research if that set price is AT,BELOW, or ABOVE market conditions for current value.If it is above what you would could sell it for on the open market it could be a bonus instead of a negative.

In Texas, you can give a gift of merchandise up to $50 in value but cannot give cash, gift cards, rent bonuses or discounts. 

I also had friends who knew another broker who specialized in renting out cape cod homes in the winter( that's a bonus on the cape as it tends to be a June- sept vacation spot).

If you can comfortably afford the 15yr alone without roommates and they are just an added bonus why not put 20% down and pay all the expenses out of pocket then use any rental to pay that sucker off faster so when you move out in 5-10 years you have a free and clear asset that will generate long term stable/safe cash flow.