Line up the coat counts. Two coats over sound paint is standard. Three are needed over bare plaster, new patching or a light colour over a dark one. It is the most common reason one quote is much cheaper.
Read the preparation, not the brand. Crack repair, sanding, caulking and priming are most of the labour and most of the price gap.
Look for exclusions. Roof coating, structural repair, rot and rendering are separate jobs. A quote with no exclusions is not complete.
Check registration and insurance. WA requires painters to be registered with the Building Services Board above a set contract value. Ask for the number and certificates of currency.
Be wary of hourly rates. They put the risk of a slow job on you.