The first thing I would suggest is to go and shop for a loan. With a 605 credit score and it sounds like slow pays on at least one trade line you may not be able to qualify for a conforming loan. With the good money that you make you should pay off your outstanding debts, the company that you are paying slow because of bad service they want you to pay slow. They are making more money on the interest than they made on the original purchase.
Now that you are in the process of getting your finacial ducks in a row before you purchase check the zoning on the property. The two units that the current owner is adding may not be allowed in your area! Does the current owner have permits for the work that is being done or are they trying to fly under the radar?
As for building another structure in front of the current building that is a zoning issue also. In my area any new construction needs a 30 foot setback and 10 feet of clearance.
Good luck and if this one doesn't make it you can always look for a duplex to get started but I would be very leary of owner added rental units.