Forgive me if some of this isn't a complete answer, I'm not sure I understood all of your questions... if you would take more time in writing your post you'll probably get more accurate help.
A 7M ECU wouldn't be the best idea as the 7M has very different characteristics.
The 1G-GTE is often considered the under dog of the MKIII and sadly not considered a decent platform for upgrades. The lack of availability of aftermarket parts (at least outside of Japan) and documentation causes most 1G owners to promptly swap in either a 7M-GTE or a 1JZ-GTE... 2JZ's an option as well. I'd really like to see what one could do if built up properly, but I wouldn't be inclined to spend the money on it myself.
I don't know if the 1G's exhaust manifold is compatible with either the 7M or the 1JZ, if not you're looking at a lot of custom work to get another turbo on there.
The ECU's are not upgradable. Toyota's of this vintage require the use of a piggyback computer to modify what the stock ECU thinks it's seeing from the various sensors or replaced with a standalone ECU altogether. Injectors will surely need to be replaced with larger injectors, if Toyota stuck with the same recipe as the other GTE engines the injectors will be low impedance, top feed... how large you'll need will depend on how much boost you intend on running and how you intend on controlling the fueling. To check the injectors use a multimeter and measure the resistance across the two contacts, it'll be between 2.8 and 3.8 Ohms if the injectors are low impedance; Or, it'll be over 13 Ohms if they're high impedance.
This site is largely trafficked by US Supra owners. The 1G-GTE is pretty much unheard of over here; You may find more information and support in a UK, NZ or AU based site, where they're more commonly imported. (I'm pretty sure the 1G-GTE Supra was never sold outside of Japan.)
UK:
http://www.mkiiisupra.net/bbs/index.php
AU:
http://www.supraforums.com.au/forum/...splay.php?f=41