1. i wouldnt advise using the rb20 radiator as it is pretty small, i would advise going to a wreckers and geting a 3 litre radiator as i have done.
2. the power steering pump wil go on yes, but u will need to put a biger belt most likely. i had to put a new belt on mine due to my car having hicas. the aircon will bolt straight on.
3.you can use your intercooler, but i would sugest trying top source a stock rb25 cooler.
4. the engine wil be a straight bolt in.
5. if you got the stock suspension in your r31 coupe i would sugest upgrading, because the stock setup is utter crap, shop around for a good deal. as for brakes they import brakes r a decent size, i would put on better brake pads. and if you can afford later on try get r32 calipers onto it. or even put slotted/cross drilled discs.
6. yes
7. the diff should habdle the power ok, u would have the r200 diff and they r a bloody good diff!, just hope u got lsd =]
hope this all helps