And before I buy each additional property they have the option to opt-out (and would retain their interest in previous investments but not invest in future ones).

As much as the right and left argue over issues... the top 1% on both sides are not trying to do away with a major tool for them to retain wealth. 

We have a decent amount of maintenance and/or upgrades we'd like to do while we are still living there (add an off-street parking spot on the double lot, repair a retaining wall, etc.), but I've been thinking about the "next step" for a few months now -- and I can't decide what's best, or what options we have.We are fine with our current living situation for at least another year or more, but I was recently thinking about purchasing another investment property once enough money was saved up for a down payment (I believe it needs to be 25% for another multi-unit).

I also allocated another about $5,000 towards legal fees and retaining a lawyer to help me understand the process of accepting investment from an outside Source.By the end of 2013 I had purchased two rental properties.

Instead look for an advisor/planner that works on a flat fee or monthly retainer model. 

Have a good attorney on retainer. 

Do they need to retain a broker to represent the transaction?

Contrary to what most hobby landlords think charging slightly below market has no bearing on retaining tenants.

So in conclusion....TL;DR - Stop looking for an real estate agent and hire a licensed real estate consultant to be on a monthly retainer or some type of flat fee.

Daniel,Qualifying for a multifamily, while retaining your current home, may present some difficulties.