Inherited tenants, no lease

[Pearce G.]

I'm preparing an offer.  Long-time tenants are paying 750/mo to seller.  Market is >1000/mo.  I'm willing to give them some time to find another place or pay the higher rent if they can, but I have to raise it soon to cash flow.  Seller's agent says tenant is month to month...willing to leave, but may need some time...but there's no lease and no property manager.  May not be any security deposit either.

What's my risk here?  If they have been living there a long time without a lease, does that make it harder to evict?  Should I require a lease (even if its m2m) as a condition of closing?  Or just require it to be empty at closing, so it's not my problem?