Stephen,
I did this with my old series 3 maybe 16 or so years ago so i'm answering from memory.
1, there is no syncro so 1st and 2nd gears are thicker.
2, double clutch, you'll get it no time. I still find myself double clutching now just out of habit and my Ser3 went in 1996.
3, 2a box and bell housing will fit, straight swap.
4, I think i just replaced the ser3 clutch at the same time. I'm not sure if it was any different to a 2a clutch. I remember changing the clutch cylinder push rod for some reason but it might have just been due to wear.
5, I can't remember the ratios being different, or noticing any effect.
6, I had no probelms at all, can't remember any pitfalls.
Cheers, G