Joth

A friend of mine bought a B4 twin turbo engine + tranny to pick apart for use in his rally car, the left over item is the autotranny from the B4 that he said he would give me as a spare for my JDM S/Tb if ever i may need it, so i'm looking around to try and find out if it is indeed a suitable swapper, or if i should just pass on this gracious gift and just let him sell it.
What code should I be looking for on the tranny and where can i check it to find out what year, specs, ratios etc are in it? Is there some resource with these codes online?
Are the auto trannys able to replace and interchange without any involvement with the main ECU?

Also what is the issue with final drives? Is it that the final drive on the front diff built into the gearbox, needs to be the same as the rear diff ratio? i.e. it needs to be the same as my existing tranny, or one may change the ratio as long as the rear diff is replaced with one of the matching ratio?
Im quite green on these issues, so thanks for any tips.