I host a lot. IMHO the food is the least of your problems op unless you live in a mansion!
Is the weather forecast good?
Lunch party indicates a sit down meal. Do you have twenty or so chairs and place settings, plates, cutlery, glasses, and
enough tables and fridge space?
Eighteen is the max number of people I can cater for at home and that’s with a lot of planning and time and a second fridge.
If it’s sunny you might get away with it! And you do fork food so that people can eat standing up.
Or could your local curry place help you out? Delivered hot to eat immediately. Two vast vats of curry; one veggie and one not? A stack of naan bread? A huge vat of rice? Ask to borrow ladles?
Depends on how many children are coming?
You might have to order plain chicken tandoori skewers for them?
Make up jugs of Delia’s ice tea?
Plastic trugs or wheelbarrow full of ice containing beer and soft drinks, water?
Buy some bamboo bowls and sporks.
Even then you might need to borrow some stackable chairs?
Ice cream cones for afterwards if you have freezer space?
Otherwise I’d be ringing around to book a reservation for my nearest pub lunch. The food and drink ingredients for eighteen will probably add up to the price of a set menu nowadays anyway.
Other suggestions:
Order in pizza (do the budgeting first)
Make massive salads?
If you have a decent sized oven, cook lots and lots of sausages and spend the weekend slowly caramelising onions (takes a while) and buying condiments, pickles, and bread buns?
Someone I saw on Instagram borrowed all of her friends’ slow cookers and had about six of them set up along one table up against a wall bubbling away with various dishes inside? Guests helped themselves.
Otherwise agree with PP’s old cuts idea. Roast all weekend. And cut up and serve with salads on Monday. Need enough fridge space though.
Ring all of the guests and ask them to please bring a piece of cheese each and you provide wine, salads, crackers, baguettes and pickles?