The roofer became irate and threatened a mechanical lien against him.  

And then for due diligence, you will want to understand the age of major items, roof, mechanicals, appliances provided by landlord, etc. 

And ask them to better explain the mechanics than I or to provide loan estimates with and without seller paid concessions.  

(this avoids him sweeping the common areas while you are doing an eviction and it would cancel it)- waiver of mechanic lien and that he was paid in full- and you don't want him as sub, just 1099 employee.

The difference would something like that between a mechanic and an automotive design engineer.I don't think the law "requires" you to take every expense or deduction, it allows you to do so.

for example if they had judgements, liens, mortgages, pending law suits, or mechanics liens.

It seems easier for a professional inspector to find the problems with an apartment building than a mechanic inspecting a car, since so many vehicle systems are sealed.

I'm in my late 20's and a Mechanical Engineer at Apple. 

I have been an aircraft mechaning for 20 years.  

@John DombrowskiA Solo 401k is simply a means to diversify tax-sheltered retirement savings, and does not provide a mechanism for you to draw income unless you are over age 59 1/2.An alternative tool that you may wish to evaluate is a Rollover as Business Startup (ROBS plan).In this plan format, you can establish an operating business that you are directly involved in and can draw a salary from the business.