The kindness thing you can do is to provide so much notice as possible so they can make a decision and plans as well.The fact is:  you're running a business. 

After they refused to sign a lease, I had my lawyer serve a 90 day notice to vacate which then lead to further problems including them refusing to pay rent.

Since it's a turnkey provider who is selling this property, they have not given me the seller's discloser notice saying that seller has never occupied the property.

He gave a handwritten, scribbled notice saying he was leaving by the end of the month.

I never receive any redemption notification or other notices regarding the sale of the tax deed. 

Pamela, generally if you're on a month to month, you only need to give the amount of notice required by law in your state. 

I noticed you mentioned If you plan to retire somewhere else , you could sell and do a 1031 exchange for that property.

Provide proper notice of non-renewal near the end of the lease term and then sell when they move out.

If not, what I would do / have done is take the letter to the tenant directly after close, explain that this is now your debt, and offer to waive it if they sign over the property ASAP (according to whatever notice you have to give). 

We’re you sent a notice that your mortgage was sold to a new lender?