Try to start with a car that has EFI. Otherwise you will need to re-work the fuel system. (An EFI car will need a bigger pump for a gze but at least the lines and tank will be OK).
Apart from that the SX/GTi has a nicer dash with oil pressure, battery voltage and a whizzy tacho. Its pretty well laid out. You might want to install one of these.
Depending on if you care about looks or not then the SX/GTi has a few minor add-ons. rear hatch spoiler, front splitter bar and nice painted bumpers. I'm not sure if the suspension is any different. I've got a '90 SX and I've never compared it to a standard rolla.