See if your brother's store has an affiliate Lexus dealer or one they're friendly with,have them pull a service history.
I worked for Lexus and from what I remember -
Front calipers - look for pistons seizing,also happens on 4th-gen 4Runners with the larger brakes (uses the same calipers).
Rear air suspension - there was a TSB to replace the compressor and solenoid valve,but it's usually faulty height sensors when a GX came in with issues.
Driveshaft clunk - customers would feel a clunk when slowing down,there was an TSB for an updated driveshaft.
My parents have a 07 GX,it was a lease and they bought it out when the lease was over. 15k sounds like a good deal and mileage is pretty low for a 10 year old vehicle. And the color looks more like Pine Silver than Dorado Gold, even the carfax link said "Vehicle color noted as Silver", the ones I worked on always had tan not dark gray interiors.