When you're less involved, costs always add up and when mistakes are made (and trust me there will be plenty) by your subs or contractors, it's best to catch them early on than wait until they are all the way done to have to tell them they did it wrong.  

I want to ask you a question/scenerio - please do not take it the wrong way- I am doing this to help you - as I will explain in a second:What books have you read and online forums have you participated to buy the 2nd most expensive thing in your life...your car?

No matter how you play it, it isn't even a deal.In my opinion and from my personal experience (I may be wrong)- I think you are intrigued that there is a possible deal with seller financing.

When we see actuals or receive proformas w/ numbers greater than these estimates it usually means there's something wrong.  

Things will go wrong, things will take longer, you'll probably go through a few contractors.

Or am I going about this the wrong way?

The marketing and knocking on  people's doors seem to me very time consuming, I might be wrong.

Either it is worded weird or he is just flat out wrong.

You can do it, but there's problems for sure if you do it wrong.

@Jacob Tomko Parts of Texas have clay type soils and the foundations need maintenance.