This question and the answer can be complicated without knowing your long term goals and investment strategies.You should develop a plan identifying all your goals and what investment vehicles you will use then it would provide you the answers to needs. 

This sounds like your situation.As @Allan Rosso noted, a partnership or LLC agreement seems to be the best vehicle. 

Hard money is a short term vehicle and not something you would want to use to acquire a rental property for example. 

When tenants apply to rent any of my properties, I require them to list their vehicles.

You can obviously modify it to ask for make and model of vehicles. 

As a landlord, should I request the current tenant, without violating the law, to update me annually with new personal information like new owned vehicles license plates (when tenants changed their cars), changes of employment, emergency contacts, or someone on the lease turns 18, etc.?

Lets partner up to allow my success be your "good stewardship" and vehicle for more cash-flow!

Drive through the parking lots in the evening and look for company vehicles. 

Become good at finding deals: with good margins (just because it is off market does not make it a good deal). splitting the deal: meaning splitting profits (rental or flips) but in this case bc you lack the funds : flipping just acts as a vehicle to acquire more funds and then do what you want: which is buy rentals. 

I do not believe he is isolating, as I went there two days in a row (one time to do some landscape debris cleanup, another time I had to service an adjacent apartment with a bad window operator) and both times his vehicle was not in the parking lot.I am also not sure about informing others.