The usual extra info ...
There are 51 events (48 Trophy + 3 TT) and the rewards 25-50-75-100% are split 7-11-15-23GC. You will need all three cars for 100% and only the Mercedes McLaren can be upgraded enough to complete the series, but with only the fully-upgraded Porsche you can get past 75%:
(or likewise just the Ferrari upgraded to a similar level)
The final three cup races are PR62.3/63.1/64.3, and I believe it costs 190GC more to upgrade the Mercedes McLaren enough (over and above the LTS-winning cost)