There are lots of free resources that track supply and demand data like CBRE, MFN, Housing wire, Zillow etc.

If this is relevant for your market and operations: Assume that you will take home 1/3 of the commissions earned from a sale (which BTW is more like 2.7% per sale, NOT 6%, and really not 3% if you're taking the long view of the data). 

I know they have loads of data and there is a business reason they are doing that.

This data was collected from the US Census, which conducts a monthly building permit survey for every US metro. 

There are a ton of data centers in loudoun county and prince William.

Ultimately I think this is a sound way to go and drafting a lease from the ground up seems like needless rework.The key things you want your lease to be are:  compliant with your local Landlord Tenant Act rules  Enforceable I'd go with the simplest solution to achieving that.

This will help you to stay in track and achieve results.

@Jacques Edouard Dessejour LapierreThe appraiser is going to have access to MLS sales and other data.  

Some data supports that my parent's generation (the baby boomers) are STAYING PUT as that generation is approaching retirement age.

I'm leaning more towards Northern VA due to the relatively friendlier landlord laws vs. those in DC, the job growth due to data centers, Amazon, and warehousing in that area, and the relative affordability.