Our local statute specifically says: "the tenant shall not be considered to be absent from the dwelling unit without notice to the landlord during any period for which the landlord has received payment of rent."

Buyer might not notice, but Buyer's home inspector will definitely notice.I bought one of these with a property in 2013.

No one will notice.  

Should I send a notice to quit a month before they move out?

I don't know where you live, but in California, I would immediately serve them with a 3-day notice to pay or quit. 

The lease clearly outlines the tenant responsibility and I'm notified of past due accounts which can lead to a 10 day notice to comply or vacate.

Another thing that makes the forum very hard to use  Yes I have noticed the number of clickbait ads on BP lately and was like whoa.

.), the tenants will need appropriate notice of the change in terms before they can be asked to sign new leases. 

If they continue smoking despite the warning, you can then serve them a formal notice addressing the problem.

I've noticed in my area that houses that have been converted into 2 units tend to sit on the market for much longer and sell for much less than traditional single family houses with similar sqft and finishes.