Meg,
for gearbox and other toyota parts, keep an eye on Toymods.net Also you could keep looking at EBAY. Personally I am going to run a 4AGTE with a early model W55 Supra Gearbox. However this required me to buy a special bell housing to get the engine and gearbox to fit together. You can get bell housings for anything from 400-650 depending on make and quality. Have a look at the toymods site, they did a group buy not long ago.
But you could run a T50 or a W50, but you may end up breaking them. If you speak to a local import company they should be able to source a RWD bell housing and box from a AE86 Sprinter for you.
For other 4AG/ZE info you could have a look at twincam.org There are heaps of other sites out there.
Computer? If you can get a hold of a standard one you could run that. I have herd it will do the job. But for the money you will end up spending on your car you should just get a aftermarket ECU. I am going to run a microtech. Why? Cheap and they will do the job if tuned correctly. Some say that they are crap but that is their opinion. Ask your chosen workshop for their opinion.
Tail shaft? Get one made to suit.

Hope this helps
Nick