get it checked out for ccjs etc, You won't get a guarantee but should pay less accordingly. You can get the AA to do a mechanical check for you...of course this and the registration (ccj) check will add to your costs. Yes get a receipt for what you pay, you might also state on the receipt that the car is in good running order but if its a retired couple without much money your recourse if it falls to pieces will be zip. Ask to see the service history (stamped book) and receipts for any large items, (engine, gearbox etc)
That's my lot, I am sure that other folk will have stuff to add