Interesting question...I think there are three main components that are required;Knowledge of or the afforded presumption of knowledge of the ability to manifest a desired outcome.A desire to communicate ones knowledge.A participant willing to receive that knowledge.Almost sounds like the components required of a mentor mentee relationship.Most get caught up negatively labeling the Guru status due to the hype factor and dream factor of both participants.Two components that when put together by unscrupulous individuals can cause disaster.There are some very good Gurus out there and then there are some that aren't so great.

Mailing is a component of marketing; just another medium, if you will, to connect two or more people to transact together.The only days I don't receive mail are days the post office is closed and it's not delivered.

I'd say the critical component to securing a worthwhile multi family investment (and any real estate for that matter) is timing.

The most important component for me is the amortization period.

*Realize that the seller is only one component of the deal.

The PMI component adds around $200 to my mortgage.

There's no guarantee the populous wanting to visit your vacation area will always have the discretionary income to vacation e.g. cyclical economic conditions.I would imagine your wife and father in-law though may have a pretty good handle on the risks associate with vacation rentals and how to minimize those risks.Getting back to my original point, if your early business plan requires a method to raise capital and you can tolerate a little higher risk profile in your investments then buy early investments that have an intrinsic value component to allow for an increase in the equity above and beyond the ingoing cash flow.You are asking the right questions.

I suggest you continue to read and learn -- that is a key component in reaching your goals -- Knowledge.

Networking is really important because every RE deal has LOTS of components and people involved. 

Agree that criminal checks can be done without SSN, it is the credit check component that suffers.