We renovated the existing structure and added an addition with more apartments and garages.

Benefits to invest in his business for some growth capital. new equipment purchase, and consolidation/restructuring of existing debt.I dont have any financial experience in regards to a business, and  just based on his explanation, something doesn't make sense with his income-debt(i dont know what exactly which is why i am here) My question is : " where is a good simple resource i can use to help me make sense of his books?"

Yes, what you are describing is called purchasing a property subject to the existing mortgage, or Subject-to for short.

I plan on doing that in a few years, but right now I was wondering what other options exist?

At least that'll blend to 12.5% over 2 years.Referral Fee: Talk to your existing tenants and tell them you'll reduce rent if they know somebody who will move in. 

It's a common tactic used by get-rich-quick gurus.Some options do exist but they are difficult and rare.

Looking at possibly just buying the LLC with owner financing their "equity" and me(only member of the LLC after purchase) continue making existing mortgage payments. 

If I wanted to update the principal balance and terms of the note, is that something I could just add to the existing documents as an amendment?

Does there exist a configurable, virtual offer template somewhere?

I am concerned about the market saturation since there seems to be a lot of these available or being built.