Hi ?
First of all welcome to the club .
In answer to your questions:-
1. You can get both.
2. Yes, I have an auto and it has been BPU'ed so running around 370 rwhp and from what I have read on here a standard auto box is good up to around 550 hp.
3. In mine, at 90, on the flat about 2800 - 3000 rpm.
4. I get around 18 - 20 mpg around town and about 250 per 3/4 tank (always fill up at 1/4 left). If you play a lot with a heavy right foot and the car has been modded then you can use some serious fuel.
5. I have a UK spec car so it has steel turbines and J spec cars have ceramic ones. Steel ones are more robust but the ceramic ones spool up a bit quicker, there generally isn't any issues with either, both are good.
6. There does not seem to be any problem getting hold of either new OEM or good second hand spares from this site.
And finally though I have answered your questions I am not a car boff, thats just what I have found. This club and its forum's are one of the best out there, all the info you will ever need on a Mk IV Supra is here, especially if you are a paid up member.
Have a look at the classified adds on this forum to see whats for sale, everything from 230 to 650 BHP.
Go on, go on, go on, GET ONE
All the best.
Bob. :d