What rate should I expect to pay on owner financed 2nd mortgage?

[Eric Hathway]

I am considering a 11 unit building for $700k in NH and only have 10% down. My bank requires 20% down so the owner said he would give me a second mortgage at a rate of 10% for either 5 or 10 years. Is that a high rate or a good deal? Cash flow will cover it, but I don't want to be taken for a sucker.