I have an 01' GS 300 with almost 50K miles... I read all those Lexus brochures about check ups and tune ups, but I don't believe their $$$-sucking marketing. Anyways, I wanted to know what essential things I should do to my car to keep it in tip-top shape...

Should I change the timing belt?
What is the best transmission fluid to use on a GS?
Should I change the water pump and the fuel filter?

Thanks.

Timing belt and water pump are done at 90k. There is only ONE tranny fluid you should use and that is the Type IV fluid from Toyota. DO NOT use anything else!

No timing belt and water pump till 90k.

I think there is a 60k service recommended for the GS. Probably includes all fluids being changed, check your air filter, maybe check your ball joints. Your car probably does not need more than standard fluid changes depending on the maintenace history. You have pretty low mileage, not much to worry about.

did you buy it used? if so, you may want to check if the prior owner did all the scheduled maintenance. if you don't know, i would change all the fluids just to be on the safe side.

all the big items:timing belt/water pump/spark plugs are done at 90K.

I disagree with this statement. Amsoil ATF has provided above expected performance / wear quality on my Trans.

I got my transmission fluid flushed at Express Oil Change 6 months ago. I wonder if I am in trouble?!?!

It is definitely something to look at.