Hi Terry major things which was missed above was fluids and belts. Engine coolant, most important to have changed on a highish mileage car is Automatic transmission fluid and differential. Cambelt on GS430's i beleive last 100,000 miles so check the history if this wasnt done at 100,000 mile service, means it will need doing with the idler and tensionor aswell. and why not have the drive belt done aswell whilst thier there. If water pump condition looks good during cambelt change then that could be changed at a later time but if you can have that changed as part of belt change then do it for peace of mind.
also if you can run the car on high octane fuel for about a month (Shell Optimax 99, Esso 97, BP Ulitmate 97) as these contain cleaning agents which cleans your injectors, fuel system, valves, and the engine as a whole this will def make it perform better than the volvo T5. dont forget GS's are refined cars so you wont feel the speed but you will be going very very fast!!