Anthony hasn\'t been around for a little while - I\'ll have a go.
Going the 1st gen. is a great idea, it is much lighter than the FD - and you will be able to run pretty good times with it (along with your engine combination).
In standard form, it should be able to get into the 12\'s without too much hassle (and around 1BAR of boost).
As for the rear end - it is quite a dilemma, I personally would spend a little bit more and have a 9 inch installed. You have a great choice of ratios and it will be very difficult to break/axels are also strong etc..
Some guys out here run the Mitsubishi Hilux Diff\'s with pretty good success - but the ratios are limited.
The gearbox you are running will be quite strong, if it is in good condition you should be able to run some pretty good numbers.
As for the engine, I would stick with what you have got for now - is it running Microtech? You can still run into problems like boost spike/creep pretty quickly if you are running a straight through exhaust (I use to run a 3inch on my Series 4 turbo engine, and had leaning out issues!).
You will probably need to run a larger injectors/bigger fuel pump (I\'m assuming it would have an aftermarket fuel pump in it of some sort?) - put a fuel pressure gauge on it also.
Also get a good tune into it.
Companies like http://www.pacperformance.com.au
sell Fuel systems, which would be perfect for what you are doing. Link here: http://www.pacperformance.com.au/content/lang__en/tabID__3931/Parts/Fuel_Systems.aspx
(makes it very cheap for you in £\'s).
Clutch maybe an issue also.
Get onto http://www.ausrotary.com
and have a look to see what some of those guys have set-up (there are a number of Series I, II, III RX-7\'s with 13B turbo\'s).
I actually can\'t believe that there aren\'t more 1st gen\'s running on the strip there, compared to an FD they are a very cheap car to get running some serious numbers.