For tenants in illegal units,  they can literally sue you for all the rent they've ever paid and win.  

You can build your own for your property type in a simple spreadsheet.

I don't want anything overly complicated since it's a fairly simple deal. 

The guy I spoke to at the city literally told me almost anything would be possible or could be changed so long as I apply for the right permit.

Ok, so I'm going to recommend a product from the Dollar Tree - seriously, I own no stock or anything - but it has literally been the only thing that's worked.

If you're just talking about him getting money from his 401(k), that's as simple as borrowing through his 401(k) custodian  and lending you the money. 

When we got our proof of funds letter from the hard money lender we literally turned it over and submitted it with all the contracts to our realtor, who then submitted it to the seller with our earnest deposit.

My way is simple I will pay you X as Y dollars per month with the first payment due in 90 days."  

And I probably know exactly the thoughts, challenges, questions, and information overload, etc. that you might be feeling and experiencing right now.And I know the "iceberg" and the "elephant" seem very daunting right now.What helped me go from buying a duplex to buying 100+ unit deals is boiling down this business into 2 simple things, Deals and Dollars.I share more of this approach in a thread if you want to read, I quit my CPA Job to buy Large Apartment BuildingsTo summarize though, the road ahead for you will most likely have many twists and turns, challenges and setbacks.

I don't know in which direction (literally) to go.