Right 3rd time of trying to write a reply ! Stupid phone,
I wouldn't like to say about the belt intervals, maybe give your local subaru dealer call see what they say, i'd be sitting down when they give you costings, however a good independent will do it for at least half the price.
The rear diff is 75/90 and I think the auto box uses dexron 3 atf fluid, again I don't know much about auto's, but www.opieoils.co.uk are a good place to start and we get a discount, again with the orange light may be an over heat warn light but a Google should give better indication,
S/tb is sport turbo and manual t/tb is touring turbo and auto's may have different interior trim, do you have any wood in yours ?
As for rear bushes there are loads, at least 16 so import car parts will advise you, could always polly bush it but they are more expensive.
This is the majority of the rear end
http://importcarparts.co.uk/parts-info.asp?id=10981
Obviously this is sti group n stuff so you could get oem rubber cheaper,
Then you have droplinks arb and other smaller bits,
I stuck a whitline 22mm rear bar on but I knew somone who was breaking a classic sti so snapped it up cheap, a bar from a classic impreza of the same year should fit and is alot thicker, they can be had relatively cheap and are on the bay.
There's loads of aftermarket ones around too just depends on how much you wnat to spend