r200s come in some 280s, and i wanna say all 280zx's. An r230 is an even stronger diff and the q45 you mention is one of the cars it comes in. Ya know, an Infiniti Q45. The r230 also came in the z32 300zx (90-96.)
As far as there being different r200s, i would say they are talking about the diff ratios they have. Now onto the z31, z32 mess, these are chasis codes for the different Z cars
s30 (1970-1978)
s130(1979-1983)
z31(1984-1989)
z32(1990-1996)
z33(2003 - )
(if any of this is wrong anyone feel free to correct me)