The usual extra info...
There are 54 events (52 Trophy + 2 TT) and the rewards 25-50-75-100% are split 5-8-12-18GC.  You will need both cars for 100% and only the S2000GT can be upgraded enough for the final two cup races.
For 75% you'll need one of the cars upgraded to PR13.4 or thereabouts, but if you already have the Silvia fully upgraded then you can do all but those last two cup races and the S2000GT Showcase and TT: