Keystone's hourly math is the real constraint here. At $500 a month, an E&O premium of say $1,200 a year eats two months of revenue, and that's before an entity, an accountant, and whatever design or landing page tool you end up paying for. One client at that price doesn't support carrying insurance. Three or four does, which is why the small shops that survive either raise the floor to $1,000 to $1,500 a month or move to percentage of spend with a minimum.
The exposure you should think hardest about isn't the ad rejection, it's the copy. Wholesale creative tends toward "cash in 7 days, any condition, no fees." If he doesn't close in 7 days, or he assigns for a fee the seller didn't expect, the complaint attaches to the promise. Ask now whether you write the copy or he approves it in writing, and keep the approvals. Whether that shifts any liability is a lawyer question, and several states regulate what a buyer of distressed property can say in advertising, so his attorney should look at the claims before they run.
Practical thing you'll hit in week two: he'll want leads in his CRM, and the Meta lead form connection breaks whenever a token expires. Nobody notices for four days. Set up an email copy of every lead as a backup and check the two counts weekly, because when leads go missing the client assumes the ads stopped working and the invoice conversation gets ugly.