Writing a offer in which seller maintains possession

[Luke Boff]

Hi everyone. I currently have a client that is interested in writing a offer on a shared wall condo. The currently sellers want the sale to be contingent upon them finding suitable housing. In order to avoid any hiccups from delaying the sale I am wondering what the best verbiage is to use when writing the contract. Should I still make the closing date 30 days from acceptance and then then allow the seller to have possession for so many days after closing? If this route is taken would there need to be a lease written up? If someone could help explain the best course of action for this deal that would be greatly appreciated! Thanks in advance!