Duplex opportunity- on a private well

[Cameron Miller]

Hello I am currently living in a 1986 built 2 bed /2 bath, 1 car garage with full basement per side duplex I plan on buying in central michigan. This duplex has a well that comes into the basement in my unit then runs through a water softener then supplies both units. I recently moved in and the landlord just mentioned that the water is well water and only to be used for dishes and showering and I need to buy bottled water for drinking and cooking. Because i am planning on buying i asked  the landlord if they have ever tested the water and they said no and theyve owned it for 3 decades. ( he lives nearby with a well as a source for water and says he never drinks it or worried about testing it and just buys bottled water to drink). They have been a successful landlord for decades and never had issues with this. For me  as a future landlord doing a house hack with a well for both units as a source of water, what are my responsibilities in the future for testing the well . Do I need to supply potable/drinkable water for both units or can I just tell the tenants to use it for showering and laundry and dishes only and to buy drinkable water ? The reason im asking is because the water tests are like 1000$ to test for PFAs , nitrates, bacteria, pesticides and pharmaceuticals etc. Would it be worth it to 1) buy a whole house RO water filter with a UV light with costs like $2000 or 2) do they water test for 1000$  annually then buy a specific water filter for like 2000$ which would come out to like 3000$ 

3) or continue to just not test it and tell tenants not to drink it

it just seems ridiculous to spend 1-1.5k annually on water tests is there a better way to deal with this

please reply if you currently manage rentals or own rentals with a well water supplied to tenants. 

Also my property is like 1 mile from a landfill which raises my concern for well quality but the area is like B to B+ I would say with many duplexes on wells in a country like setting. The water doesnt smell and tastes fine in my opinion.  Thank you (: