There are 20+ real estate CPAs commenting in this forum.

It is a frightening experience but if you have the right team in place you'll be confident in the property you chose to close with and it will go smoothly.

I’m looking at purchasing a quadplex in New York City, where all four units are rent-controlled, with two units vacant and two still occupied by rent-controlled tenants.

And since you hold the property with your wife in a trust that makes this a related party scenario. 

This in turn can open up what homes you can buy since the numbers will make more sense when putting 3-5% down instead of 20%. 

Is there a better way to find out how many existing units are there in each town, or do I need to drive to each facility and ask?

It sounds like a single family house where you will be living in the house(personal use).Therefore, you may not be getting the tax benefits you think you are.Consult an accountant.I am not sure why your sister would agree to this as well, she is losing $170,000 in this transaction and she faces the possibility of you increasing her rent.You also have to charge market rent or close to market rent or it is all deemed personal use.

redid this as it ran together in Nates posts.

Do whatever that is some more.

Real estate has so many tax benefits and credits available, that I always tell people it's crucial to have a CPA that's extremely knowledgeable in this area.