Get yourself 2 brokers, and watch them compete. Then, when you have found an aircraft that interests you, hire a good maintenance expert to supervise the pre-buy process.
Unless you are an Gulfstream expert and a money-man yourself, there's NO way you can safely avoid the broker fees associated with a sale, no matter how simple and straightforward.
As for the MXX-consultant, the 20'000$ you are going to spend on him are absolute peanuts compared to the money and aggravation he will save you.
There are plenty of "cheap" G550s out there. Finding one with the warranty programs paid up, that will stay compliant for the next few years without Millions of upgrades, and with no hidden issues, is not as simple as it sounds.