As I posted in Chat, it's a 1991 model, despite being built in 1990. Also, GT measured C-C on the seat tube, so the model would have been advertised as a 20" originally (20" C-C and about 22.5" C-T)
I don't have any idea on price of the Suntour parts, so I can't really help on prices other than what we've previously discussed via PM.