I'm renting out the renovated home and have put up a privacy fence that defines the yard, leaving a well defined adjacent lot.

Step one would be defining your criteria.

Every component of a house has a finite design life and at some point needs to be replaced.

Does anyone know how Westchester County defines an in-law suite? 

Some would say it's working IN my business, but as far as I'm concerned, I am learning a LOT of lessons about the structural components of real estate, how the work is done, what material prices and labor costs to expect for various components, the scope of work that I hadn't accounted for, small details that are useful to know when looking at the next property, managing contractors and subs, taking ownership of shortcomings and finding creative solutions to tackle problems, and equally important; Having an absolute blast while I'm at it, fueling this passion! 

Survivorship in joint tenancy defined; proviso as to partnership; unequal ownership interests.

Obviously, this can depend on how you define success but for a newbie, what should their focus be?

Since the map for you is much more wide open, instead of recommending an area for you to search I would suggest having a conversation with a realtor or a few to define what you are looking for in an area.

But first it takes clarifying that end vision, then defining the strategy.Hope that helps!

They can vary between B and C class neighborhoods however, so you need to define your criteria and know the neighborhoods well or work with someone you trust that does.