Hi Billy,
I have had a different experience with house hacking than other people here are referring to. My wife and I moved to Leesburg last year to buy our first SFH and kept a condo in Tysons Corner as our first house hack. We needed to negate the mortgage on the condo for our DTI to qualify for the house we wanted and our lender (from Loudoun) had no issues counting 100% of our rent towards the DTI without any prior land lord experience. The only stipulation was we needed a signed lease, which given your situation shouldn't be hard to get from your parents.
After considering your entire situation I'm not sure why your parents wouldn't sell the property and pocket some money but if they don't want to move and want to "save the farm" as you've said I would recommend finding a lender who will count the entirety of rent as a way of negating your mortgage payments. My in-laws live in Purcellville off Snickersville turnpike and this is something my wife and I will likely be considering as the in-laws age and my wife and I continue our REI journey. Of note,@Sudhanshu Singha is right in that land in western Loudoun is at a premium but you can get a significant discount/break on your property taxes if you sign something saying you won't subdivide or sell to a developer. I'm not sure on all the legality of that form but my in-laws have mentioned it as they have 6 acres and a few horse stalls themselves.
That's a very interesting situation you've got but I tend to believe anytime you can buy and hold real estate in this area (especially if that means keeping your parents in their house) then you should at least explore that option. Obviously your parents may just want a fresh start with some money in their pockets but if you choose to pursue the property please share the journey on here as I will likely be doing something similar in the years to come. Also if you need a lender reference who I know used all of our rent to negate a mortgage in the pre-approval process you can direct message me. Just out of curiosity where is the farm located?
Best of luck,
Paul