I’d look into historic appreciation data for the area before making a decision.

Out of the 20 letters I've sent, I've had 1 guy get back to me and we were in discussions but weren't able to find a price that worked for the both of us.The issue is that I only have a few houses on my mailing list so far (just places I've walked or driven by that caught my attention), and blindly searching every street and address # to see which houses are actually legal multifamily will take forever and doesn't seem efficient.I know the data is publicly available, I just don't know how to aggregate it in a way that would be efficient and useful for blasting out marketing mailers.Are there any websites that can pull massive lists of all multifamily houses in my area?

Have good data and comps. 

Some tools myself and others have used: - Skip tracing- Mailers- Data aggregators like PropStream, PropertyShark, etc. 

Propstream is great, you can get a lot of data off of there as well as driving for dollars which I believe is also around $99/month. 

They would never look at 99% of the 154 units that are listed on Zillow at any given time.I used this exercise to show how opportunities fall through the cracks & most investors make investment decisions based on bad or irrelevant data.

Do you rely on specific data sources, gut instincts, or perhaps a blend of both?

You learn a lot about data and analysis, renovation costs and dealing with tenants which can mitigate some of the fear of investing.

First step is looking at the data of your comps.

Once you find an iBuyer, the company will determine an appropriate purchase price using market data.