I have 1994 import LS400 and at 17 years old my hoses are in perfect condition and serviceable so why your mechanic has suggested changing them all I don't know,are they collapsed,leaking or falling to pieces?
There are a lot of rubber components on and around the engine and to replace them all would be time consuming ( so expensive) and expensive to buy in the first place.
Check the main coolant hoses yourself are they showing and signs of deterioration for example cracks in the rubber?
Drive belts should be replaced at the recommended service intervals there are only 2 the serpentine belt and the timing belt of the 2 the timing belt is the most important as if it breaks it's basically new engine time.
Other rubber components that do wear out are the motor and transmission mounts but again a visual inspection should determine if these are good or bad plus you would have noticed excesive engine movement should they have gone.
It sounds like your mechanic is asking you to have unneccesary work done if in doubt seek a 2nd opinion.