things to be aware of when purchasing a unit that has a tenant

[Mei Zhu]

Hi the seller has a tenant who 's lease expires december. he offers two options:

1) sign the contract now and wait to close when the tenant leaves in december .

2) close now and keep the tenant. 
If we choose either option, are there things we should be aware of?

I plan to do my own background, credit and eviction check. 

Seller saids that tenant pays cash first of every month and has been a very good tenant. He flips properties. 

thanks