IRS = Independent Rear Suspension. You'll be able to tell by looking at your diff.
Regarding a diff and gearbox, it all depends on what your budget is. R31 diffs are thin on the ground any wrecker I go to, plus they don't just bolt in to a 200B. If you can find a series 3 Bluebird sedan, get the complete H190 drum-to-drum diff out of that. The housing itself will bolt straight into the 200B. You will need to sort out a tailshaft, either by using a one-piece manual Bluebird tailshaft (I've never managed to find one) or by using an auto 120Y tailshaft. You might need the tailshaft flange from the Bluebird diff put on to the 120Y tailshaft, I can't remember. I also remember having an issue with the brake line when I fitted it to mine, I can't remember exactly what the issue was but at worst you will need to use the 200B brake line on the Bluebird diff.
Once you have that in, you can keep your eyes open for a better H190 ratio and an LSD. The R31 LSD will be easier and cheaper to find, but you'll have to get mounts moved on the diff and they are quite a bit wider than a 200B diff so I don't know what your options with wheels will be. The Bluebird diff is 15mm longer either side than a 200B diff.
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green200b - aka Alistair
The fleet:
Australia:
'80 200B GX - the
green200b - hibernating
'81 RX-7 - 12A extend port - daily
'74 RX-2 - 13B bridgeport - cruiser
Europe:
'93 200SX - CA18DET - tourer
Green200B has forgotten more about 200Bs than we will ever know.