Skyline calipers are for bigger discs, so the offset will be wrong. These discs will work nicely with them though:
http://www.datsport.com/R32Z_CO_DS.html
That is the only true upgrade.
S14/Z32 calipers are designed for 280mm discs so bolt on with no issues. You dont actually need spacers for the thinner discs either.
Other than that, you could go for custom bells/discs that are available for the S13 but the offset of the bell might be wrong, so some measurements of the disc offset will need to be made. You never know, they may be the same.