Max interest on a land contract?

[Kevin Mason]

My uncle is selling me a property worth $100,000 for $60,000. The terms are $24,000 down, $1000/mo for 36 months. 0% interest.

He suggested putting down a purchase price of $45,000 and with 20% for 3 years, comes out to about $60,000 total.  It would be nice to write the interest off, but I don’t want to go beyond usury rates.  I can’t seem to find the info as it pertains to real estate.  Anybody know if 20% is too high in Ohio?  Thanks!