When the day to sign the lease finally arrived, she called me and said she had to back out.

I'm sure the struggles by which the TMPs finally discover Furnished Finder is a whole other post!

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

Finally, confirm if there are any existing service contracts (for landscaping, pest control, etc.) that may need to be transferred or canceled.

I heard that phrase a while back (I think it was a BP podcast) and it stuck with me.This has been a long time coming, but my wife & I finally worked ourselves to a position to make that decision. 

Have good data and comps. 

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

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?

After that, I would encourage you to go to your county recorders office / court to request data yourself.

Finally wealth creation, this I'd do it for.