Ask other sailors which broker they have used - more than once preferably.
Budget is up to you - a good broker will stick to it unless it is totally unrealistic to get what you want for the price you want to pay. You have to like the boat and its price. If you don't you don't have to buy it. A broker that is paid on commission wants you to be happy. Someone that agrees to a flat rate isn't as motivated in my opinion.
You don't have to trust anybody but it sure helps if you can find a broker who has pleased other buyers multiple times to trust.
Living aboard in Victoria Harbour