Help with terms on "owner carry" or "owner financing"

[Erica France]

I'm about to make an offer on a raw land purchase, which the owner will carry. What would you suggest for terms? 

Some context: There is a buyer's agent, and a seller's agent. The owner purchased the property in the early 2000s for ~$200,000 without the help of an agent and then learned a few "prohibitive details" about improvements that would need to be made to the property (cost to run power, drill a well, road work, etc.) They backed off their plan to short plat it into 3 parcels; meanwhile, their kids moved out of the area and suddenly they didn't have a compelling reason to keep/develop the property. They aren't in a hurry to sell, don't need the money, yet want to make as much of their money back as possible. The land has been on the market for 2 years, with no known offers. Asking price= $139,000. 

Here's my starting point: $125,000 at 5% interest, 10% down, 20-year note, no balloon payment, and no pre-payment penalties.

I realize that location, market, size of the property, improvements, monthly payment limitations, etc. all play into the offer and that is too much detail for here, so perhaps I could use the most coaching on what principles to follow and whether my starting point seems reasonable, insultingly low, or if I should get more aggressive.

Thoughts? Suggestions? Thanks in advance!