End goal is to eventually scale my real estate portfolio and make real estate investing my main source of income.

BTW, Theres paint shortages so confirm that they have a good source even SW has been tight on some of their products.

My grandfather willingly let a friend of a friend move into a studio apartment ADU in his backyard.It has been a couple of years now and she has piles of junk inside and outside of the unit and has also been throwing food scraps into the yard and into the pool.I believe her plan was to outlive him and acquire the property through adverse possession.Long story short, I'd like to hit her with a 3-day notice and start the eviction process ASAP.Is a 3-day reasonable in this situation, in San Bernardino county California?

Definitely if the cash flow is the primary source of return, there are numerous other investments that are more passive (ADU additions are far from passive) that are likely to produce better returns. 

I’ve seen it done and it’s food for thought as you market this property once you sign on your newest clients.

There are webcasts and tools that will help you find deals to get started and analyze them to be sure they are good ones and not lemons . . . very inexpensive source of education and one "bad deal" woudl more than cover what you woudl have paid for an annual membership.

I believe the overflow came out of the washing machine drain (first noticed there by the tenant), but I don't think that rules out the kitchen sink as the source. 

We're in the Blue Ridge Mountains so people come here for the hiking, outdoor activities and cities nearby that have great breweries and food.

Just some food for thought, but I typically find that clients get hung up on all the shiny objects in REI, and to be honest there isn't a better return on investment than house hacking!

If you're going for traditional financing, NO lenders will accept it.For primary purchases, you can get around it by asking family for gift funds, then "gifting" the relative the cash later, so the lender can source the funds.But for investment property loans, cash gifts are not allowed.