TIA Bruce Kirsch has a great course on underwriting.

As Jay pointed out all this reads of what I'd call "par for the course" in D/C class.

referral of course! 

I prefer to have the least amount of banks as possible.Create a bank account that you already have a relationship with(Credit card or mortgage)The least amount of bank accounts is also ideal.

I would stay away from section 8, pull credit and background, and call referrals, and look at pay stubs for everyone who applies.

Plus, if you aren't able to get approved with what you've got now in terms of savings, credit, etc. you can learn on what you need to qualify so you know what to be working towards. 

Sadly there’s a lot of bad actors out there peddling similar courses and schemes.

Since it was a structural change, you need an architect or engineer to draw up plans that make sure the new changes are safe of course. 

Your son is responsible for making payments on time or ruining this seller's credit. 

With the exception of building a trailer on the property (these are hard to finance), this sounds like a good plan AND if you are using it for your business AND your business pays more than 50% of the rent (if you have more than 1 tenant), then this is a more financeable property in the eyes of banks and credit unions as it would be consider a commercial "owner-occupied" property.