If I were in your shoes, I would most likely write them a letter.

Which makes you at the very least an unknown quantity to a broker, and at most a time-waster.Put yourself in their shoes: they're in the biz to make a buck.

You need to think of a number that appears fair but motivates the buyer and the seller to get the deal done rather than focus on ways to cut you out.Overall I think the buyer is over paying.

But other than not doing due diligence and with zero knowledge powering something improperly, it tends to appear mostly ways in which human error is not so much the issue as is faults within the materials.Sorry but I just don't believe anything you say and I think you are giving poor contracting advice.

I have to 'personally guarantee' the loan, but as long as it is never in default it will never appear on my credit reports.  

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I have no idea what future guideline changes will bring, but now there is the technological ability to actually exclude those debts, should it ever be decided by the powers that be that this would be a good idea, because the trended data is right there on the credit report (actually started appearing, in my office, a few months ago).

These 2 apartments back in the past was stated as 1 house and in the mid of the house where the stairs appear to be today , it was the house's hall, as the years passed the old owners decided to break down the hall's wall and make it practical to be 2 split apartments but they didn't change the contracts to appear as it is now and also the architect didn't change the files of the house as well.

It has strong numbers with a cap rate just below 10% and photographically appears to be in good condition. 

You have to dig into the details... you appear to be guessing at possible rents... call and ask what they are.