Within the rules that say you must keep the stock ECU box and connectors, for the money, the MegaSquirt EFI probably is your best bet.
The following thread is a discussion. HemiYoda in Sweden has added a MegaSquirt to his 5M-E (not 5M-GE) that uses a lot of the stock sensors on his engine.
http://www.celicasupra.com/phpBB2/viewtopic.php?t=1902&start=0
Erik Berg sounds REALLY knowledgeable about this and very helpful.
It works. he has included a link to Videos he has running this modified car at a track day.
He also has photos showing the MegaSquirt EFI board fitted into the stock ECU case and connectors.
The down side: No ignition timing. He uses centrifugal advance only on his engine (that's the way they came in Sweden).
The system is a Speed/Density system. I know a lot of people express reservations about not measuring airflow, but the truth is, the 5M-GE air sensor cannot read past about 2/3 of full range anyways, and the airflow door is very restrictive. For rules reasons, one could remove the flapper in the air sensor so that it appears stock.
There are tuning programs that run on a laptop so that you can re-map on the fly.
The real upside is that the system is developed by two gentlemen who are not out to make money. They want to interest people in using and knowing more about fuel injection. The systems are VERY inexpensive, especially since most of the stock sensors are used.
There is going to be a new version of the MS system out in the not too distant future. Ultra MegaSquirt UMS. It will allow the use of a wide-band O2 sensor so you can tune on the fly without having to do any guessing about how close to Stoich you are. It will also probably support a distributorless ignition. I don't know if that will be within autox rules or not.