It will vary from market to market, but most markets have a variety of public data available.

I like to be informed of the long run of the market before jumping in, and I hope to bring that philosophy into real estate.How granular can the data get, say for the metropolitan area level, for metrics such as inflation-adjusted rent (for 1 br?

They use various algorithms and public data which vary wildly.

You need data showing the difference on relatively large sample say N=10 per sf.You may make tenants happier that someone is not ripping other off.

What is the most efficient way to get this data?

It's still quite affordable for multifamily investors, the commute is easy, and the area has been appreciating over time, besides being very family-oriented.I'm a data-driven kind of broker, and I find this graphic from Bloomberg to be extremely helpful: https://www.bloomberg.com/graphics/property-prices...Also, make sure to always check NYC's crime map: https://maps.nyc.gov/crime/ You can also see crime rates on Trulia and will soon realize that many listings for sale are on "hot spots" on those maps, so be mindful of that.I would also strongly advise you to find a good broker in order to assist you in the transaction: it may give you a clear advantage over other investors as they will know how to put you in the best position when presenting an offer and may be also aware of off-market deals. 

The actuary uses claim data to determine risk.

Secondly, although you can find some of the info MPP provides online for free, it allows for it's users to access all the necessary/needed data in one easy to find place. 

Propstream, like any other data provider or software, is a computer program that uses and 'interprets' data to help users build lists. 

the data speaks for itself.Feel free to reach out anytime and best of luck whatever you decide.