I dont think finding a Supra Getrag 6-Speed will be any cheaper than soarer stuff..
But what will be needed will be the Box, 2JZ 6SPD flywheel and clutch assembly, prop shaft and some other buts such as clutch lines and assemblies.
Have you actually looked into a JZX90 Manual setup (5Spd). Im sure the're the same and will be best suited for the Soarer.
As for how much the 6SPD will handle. the Supra 6Speed (2JZ-GTE) makes alot more than the 1JZ-GTE Stock...
Have a lookie online for pcars in japan and do what i did by buying and then using the box from a import...
www.tau-net.co.jp
Car from Japan with 1JZ-GTE and R154 (Very rare in NZ and expensive) ~$4,000-5,000 (in your posession) and MOST of the stuff required for a conversion
Getrag alone ~$3,000-5,000
Make sure if you do take the car from japan track, that you get JZX90 setup as the rear casings are different and the shifters are in different positions.
Thats all i can think of for now...