8% is very reasonable, most contracts have been at 10%. You have two ends of the spectrum, on the high end, you have usury rates set by state law as the highest amount you can chearge, most are 10%. On the low side you would be concerned about imputed tax rates, too low and the IRS will impute a tax rate that should have been charged.
As mentioned above, rates should reflect the risk involved, but few sellers actually address the issues and just name a rate. 8% is fine!
Talk to a mortgage originator and see when the borrower can refinance, then go about three months beyond that (or longer)as they can't get a loan if they are in default and they need time to get the loan. Failure to do so can present problemsfor the seller too. Good luck