If she uses her security deposit for last months rent obviously I have no security for damages or other issues or any real guarantee she is going to leave at the end of the month.

Personally, I analyze the residential units as guaranteed income and don't look at the commercial space at all.

did the company that sprayed it the first time have a guarantee for a set period of time? 

If you have enough funds in an IRA the alternative would be to establish a self-directed IRA, and have the IRA buy the property subject to existing financing (if your sister is willing to do that), you personally can't assume the mortgage b/c you are not allowed to provide a personal guarantee for IRA-owned property and have the IRA pay your sister for the equity.

I was given the impression that it was a special deal for us, but as I read the engagement letter, the money back guarantee was a standard language, offering clients 30-day money back guarantee. 

They benefit by having a year lease with guaranteed income.

The mortgage is likely also personally guaranteed by current owners.

The only guarantee in the Rental Business is to get First, Last and Full Security Deposit.

Everything gets better if the net profit is 15%+, but that's not something I want to guarantee.

Usually these are jobs that are contracts for a few months and are not a guaranteed salary for 12 months.